Based on the atomic cluster theory and quantum mechanics, the isothermal curves of solid helium and solid hydrogen have been calculated. In the calculation for solid hydrogen, the spherical potential approximation suggested by Cui et al. and the effective helium atomic model of ours were adopted. Comparisons of the theoretical results with the avialable experimental data, which are all below 100GPa, were made and it was found that they are in good agreement with each other.