Finite difference techniques for the solution of the motion of an elastic-plastic solid are used to investigate the effect of rock strength and the cavity gas properties on the cavity size formed by a nuclear explosion. The material description includes the effect of pressure and temperature on the yield surface and the change of material description in the solid, liquid, and vapor phases. The results presented indicate a strong dependence of cavity size on the rock strength and a considerably lower sensitivity to the ideal gas coefficient, γ, assumed for the cavity gas. The results suggest that the cavity sizes observed in nuclear field tests can be better correlated with calculations by assuming strength parameters considerably lower than observed in laboratory tests on competent rock samples.