Ozone resistance of poly(vinylidene fluoride-co-hexafluoropropylene-co-tetrafluoroethylene) (poly(VDF/HFP/TFE)) cross-linked by peroxide was examined in water. Although the poly(VDF/HFP/TFE)surface was not macroscopically damaged after the ozone treatment in water, physical properties at the surface were altered. Fourier-transform infrared spectroscopy with an attenuated total reflection mode (FT-IR ATR) revealed that the ozone treatment in water decreased the number density of the triallyl isocyanurate moiety, corresponding to the cross-linking point, of poly (VDF/HFP/TFE) elastomer in the vicinity of the surface. Finally, a correlation between the changes in primary structure and physical properties at the surface is discussed to give a better understanding of the ozone treatment to cross-linked poly (VDF/HFP/TFE) in water.