Plasma-polymerized fluorocarbon polymer films are thought to contain a high density of residual radicals which are easily oxidized in air. The rf discharge power dependence of the oxidation rate in C3F6 plasma-polymerized films under an accelerating aging condition of 65 degrees C/85%RH was studied to estimate the density of residual radicals in the film. The corresponding discharge-power dependence of the plasma structure was also investigated by an effluent mass spectrometry. A critical discharge power was found where the oxidation rate and the plasma structure exhibit substantial changes. X-ray photoelectron spectra showed that the surface of the plasma-polymerized films prepared below the critical discharge power exhibited an excellent oxidation durability as a sputtered film of polytetrafluoroetylene. The films prepared above the critical power exhibited a high oxidation rate. An increase in -C- peaks in the C 1s spectra also suggested defluorination as well as oxidation. The secondary ion mass spectrum for the internal region of the aged plasma-polymerized films prepared above the critical discharge power contained oxidized CxFy species, while the spectra for the aged films prepared below the critical power contained no oxidation-related peaks. Concerning the mass spectra of the C3F6 plasma, high-mass species over 100 amu, including gas phase polymerization products such as C4F7 and C4F8, were predominant below the critical discharge power. Above the critical power, these high-mass species were rather decomposed, and the intensity decreasing with increasing discharge power. Simultaneously, the intensities of the small fragments such as CF, CF2, and CF3 increased with discharge power, causing about 50% of the total pressure increase. The films obtained under such a fragmentation predominant condition contained a higher density of residual radicals, resulting in a deterioration of the oxidation durability.