CINCINNATI - Two games, two grand slams. Carlos Beltran has started a very rare streak.

Beltran hit the last of New York’s three homers Tuesday night, a seventh-inning grand slam off Eric Milton that powered the Mets to an 8-3 victory over the Cincinnati Reds.

Beltran’s slam off Milton (6-5) was the sixth of his career and his second in two games. He and Cliff Floyd connected Sunday during a club-record 11-run sixth inning that set up a 13-7 win against the Cubs at Wrigley Field.

He became the 23rd major leaguer to hit grand slams in consecutive games. David Eckstein was the last to do it, for the Angels in April 2002.

“That’s baseball,” Beltran said. “Some days you feel good, some days you feel not so good.”

Beltran added a run-scoring double in the ninth off left-hander Bill Bray, acquired in an eight-player trade with Washington last week. The loss was Cincinnati’s first since the All-Star break, coming off a four-game sweep of Colorado.

Xavier Nady and Jose Valentin also hit solo homers that helped right-hander Mike Pelfrey improve to 2-0 after two major league starts. The 22-year-old rookie gave up seven singles and a pair of runs in six innings.

A first-round draft pick last year, Pelfrey was called up from Double-A on July 7 to help a short-handed rotation. He’s the third Mets player to win the first two starts of his career, joining Dick Selma (1965) and Gary Gentry (1969).

“He’s got a long way to go, guys,” manager Willie Randolph cautioned. “I wish I could throw a lot of superlatives at him. Eventually, I will.”

The Mets have won five of their last six games, improving the NL’s best record to 56-37. New York has led the NL East by double-digits since June 22.

New York’s offense is one of the league’s most prolific, and its bullpen one of its stingiest. The main concern at this point is a rotation missing right-hander Pedro Martinez, on the disabled list since July 6 with an inflamed hip.

Pelfrey, who was born at Wright-Patterson Air Force Base in nearby Dayton, is the third Mets starter to make his big-league debut this season. He won on July 7, allowing three runs in five innings of a 17-3 win over Florida.

The 6-foot-7 pitcher was under a lot more pressure the second time around, but came through again.

“Today I just trusted myself and went after guys,” Pelfrey said. “They hit some balls hard. Luckily, most of them were right at guys.”

The Mets turned double plays in each of the first three innings — Ken Griffey Jr. grounded into two of them against an infield shift — to keep Pelfrey out of trouble. He also got Milton to fly out with the bases loaded in the second, and fanned Adam Dunn on a 96-mph fastball with two aboard in the fifth.

“I reared back a little and threw it for a strike,” Pelfrey said. “I got lucky he missed it.”

While the Mets’ defense bailed out Pelfrey, their power-laden lineup pulled him through.

Milton gave up 43 and 40 homers in the last two seasons, a bad mix of flyball pitcher and home run-friendly ballpark. The left-hander is giving them up at the same pace this season.

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Reds manager Jerry Narron decided to leave Milton in with the bases loaded and two outs in the seventh to face Beltran, even though he had thrown 103 pitches at that point. Beltran missed badly on a changeup, then hit his 27th homer into the upper deck in left field.

“I went with my heart, and I got burned a little bit,” Narron said. “What can you do? I have a lot of guys who’ve shown a lot of heart, and I wanted to repay it a little bit.”

Beltran, who is 2-for-5 with the bases loaded this season, got hold of a slider over the inside half of the plate — right where Milton didn’t want it to be.

“I felt good all night,” Milton said. “It just didn’t work out. It came down to one at-bat. If I get that guy out, I end up pitching pretty good, and maybe we win the game. It came down to one pitch, and it was bad.”

Notes: Martinez is back to a full diet, four days after he was treated at a Chicago hospital for food poisoning. He’ll throw in the bullpen on Wednesday. Randolph said Martinez will throw a simulated game near the end of the week, and a decision on his future will be made after that. ... Jose Reyes was back at shortstop Tuesday. He needed seven stitches in the small finger on his left hand after he slid into first base on July 7. ... Reds LHP Brandon Claussen had a sore back, forcing him to skip his latest scheduled rehab start for Triple-A Louisville on Monday night. ... 3B Rich Aurilia extended his hitting streak to a season-high 11 games.