An equation of state (EOS) model of detonation products based on chemical equilibrium is developed. The EOS of gaseous detonation products is described by Rosss modification of hard-sphere variation theory and the improved one-fluid van der Waals mixture model. The condensed phases of carbon are taken as a mixture of graphite, diamond, graphite-like liquid and diamond-like liquid. For a mixed system of detonation products, the free energy minimization principle is used to determine the equilibrium compositions of detonation products by solving chemical equilibrium equations. The potential function parameters have been renewed and the non-ideal fixing effects of the major detonation products have been taken into account. The calculated detonation parameters in our work for a variety of explosives are well in agreement with the experimental data.