The equation of state of gas detonation products is described by Ross's modification of hard-sphere variation theory and the improved one-fluid van der Waalsmixture model. The Gibbs free energy of dissociated carbon is calculated for the most probable state, which is determined by differentiating the four states of carbon, namely graphite, diamond, graphitelike and diamondlike. The equilibrium composition of detonation products are calculated by solving chemical equilibrium equations based on minimizing Gibbs free energy. The results are in good agreement with the results based on Becker-Kistiakowsky-Wilson (BKW) and Lennard-Jones-Devonshire equations of state. The detonation properties of explosives are calculated with this theory. The results of high density explosives based on BKW, Jones-Wilkins-Lee (JWL) and our equations of state are in good agreement with the experimental data. For low density explosives our results are better than that of BKW and JWL.