Adam Birchall clinched a third successive win for Barnet.
He struck after 13 minutes when Nicky Bailey released Barry Cogan and his cross found Birchall who turned before firing past Posh keeper Paul Rachubka.
Barnet should have gone in at 2-0 up, Dean Sinclair failing to hit the target when put through in stoppage-time.
Liam Hatch should have sealed it at the end, but he completely missed his kick from Birchall's cross as Barnet failed to win by a bigger margin.
Barnet: Flitney, Bailey, King, Gross (Charles 31), Sinclair,
Graham, Yakubu, Devera, Puncheon (Hatch 87), Cogan, Birchall.
Subs Not Used: Jaimez-Ruiz, Warhurst, Vieira.
Goals: Birchall 13.
Peterborough: Rachubka, Arber, Holden, Branston, Morgan, Gain,
Butcher (Smith 66), Stirling (Yeo 55), Crow, Newton (Benjamin 78), Mclean.
Subs Not Used: Futcher, Day.
Att: 2,958
Ref: K Hill (Hertfordshire).