Philadelphia, PA – Michael Morse hit a three-run homer and game sweep of the Phillies with a 6-1 victory.
Peacock (2-0), in his second career start, gave up just one hit, walked two and struck out two to earn the win.
"I thought he was outstanding," said Nationals manager Davey Johnson. "The young man handled the pressure very well."
Roger Bernadina went 2-for-4 with a double and two RBI for the Nationals, winners of five in a row and 11 of their last 14 games.
Roy Oswalt (8-10) allowed six runs on seven hits over 7 2/3 innings to suffer the loss. He walked two and fanned four.
Carlos Ruiz went 3-for-4 with an RBI in Philadelphia's final home game of the regular season.
The Phillies have lost a season-high six straight games since locking up their fifth consecutive NL East title on Saturday. They have already secured home field advantage throughout the playoffs.
"We do have to pick it up," said Phillies manager Charlie Manuel. "It's time for us to get ready for the playoffs."
The Nationals took advantage of a Phillies misplay to take a 2-0 lead in the third.
Rick Ankiel led off with a double. With two down, Ian Desmond lifted a fly to shallow right. Second baseman Chase Utley and right fielder John Bowker converged, but let the ball fall between them, resulting in an RBI double for Desmond. Bernadina followed with an RBI single.
Oswalt began the eighth for the Phillies and retired the first two batters before Desmond singled and Bernadina doubled to make it a 3-0 game.
A walk to Ryan Zimmerman prompted a visit to the mound from Manuel, but Oswalt indicated he wished to remain in the game. The next batter, Morse, ripped a 3-2 fastball over the wall in center for a 6-0 lead.
"I was trying to go down and away to Morse and the ball kind of leaked up on me a little bit and he hit a home run," said Oswalt.
The Phillies' only other scoring threat came in the sixth, when Peacock was relieved after issuing walks to Oswalt and Jimmy Rollins. Tom Gorzelanny entered with two down and walked Utley, but got Raul Ibanez to fly out and end the inning.
The Washington franchise earned its first sweep of the Phillies since the Montreal Expos turned the trick from Aug. 25-28, 2003...The Nationals won the season series 10-8...The Phillies finished with a home record of 52-29.