In the real sea condition, only multi-directional waves or short crested waves are found. Short crested wave isdefined as thelinear summation of various long crested waves propagated tovarious angles, where the magnitude and direction generated are randomly varied. In this study, the dynamic responses of classic spar platform subjected to long crested and short crested waves are investigated numerically and compared. Two numerical simulations are developed by incorporating Diffraction theory to obtain the wave forces for long crested waves and short crested waves. The classic spar platform is assumed to be rigid with three degrees of freedom surge, heave and pitch, restrained by four mooring lines. For every time step, the mass, damping and stiffness matrices are evaluated. The equations of motion are formulated for the platform dynamic equilibrium and are solved by using Newmark Beta method in time domain. The results in terms of Response Amplitude Operator (RAO) for surge, heave and pitch motionsare obtained and compared. © 2012 IEEE.