Abstract: Based on Wyllie's time——average relation and sonic velocity log conditions, introducing three assumptions and utilizing conventional elastic wave dynamical method, both P-wave and S-wave velocities are derived in this paper. And Wyllie's kinematical model for P-wave velocity is developed into dynamic model for P-wave and S-wave velocities. Our result for P-wave velocity is in good agreement with Raymer-Hunt-Gardner empirical formula. Also, our results are consistent with Wyllie's data on velocities for air-saturated sandstones.