The stabilization of low dielectric constant SiOF films prepared by conventional PECVD using TEOS and C2F6 was improved by the N2O-plasma post-deposition annealing. Deposited SiOF films had lower dielectric constant (3.39) and better gap-filling than undoped-SiO2 film. However, their properties became unstable as they were air-exposed or heat-treated. N2O-plasma post-deposition annealing was proved to be effective on stabilizing SiOF films, which was mainly caused by the F desorption and the formation of Si-N bond near the top surface of films. However, the value of refractive index was greatly increased again when N2O-plasma post-deposition annealing took longer than the proper time. To stabilize the SiOF films without an increase of dielectric constant by N2O-plasma post-deposition annealing, one should stick to the minimum annealing time, for which stabilizing effects of plasma annealing were preserved.