C4F8/N2 gas mixtures are possible substitutes for SF6, because of its good insulation characteristics and low influence to environment. The thrust of this paper is to verify the dew point of C4F8 and C4F8/N2 gas mixtures, the dew points of C4F8/N2 mixtures are achieved using experimentally measuring and theory calculating method. The experimental data obtained is compared to that predicted from the Clausius-Clapeyron equation, Trouton’s rule and vapor pressure equation. The results show that the experimental data verify well with the predictions. So the prediction method can be used to predict the dew point of C4F8/N2 gas mixtures for different charging pressures.