In this paper the Inter-Channel-Interference (ICI) caused by Doppler spread is analyzed in the discrete form for an Orthogonal Frequency Division Multiplexing (OFDM) system. By theoretic derivation it is verified that the ICI power of the flat channel is the same as that of the time dispersive channel with the assumption that the flat channel is a normalized Wide-Sense Stationary ( WSS) stochastic process and the time dispersive channel is a normalized Wide-Sense Stationary and Uncorrelatd-Scatting (WSS-US) stochastic process. Furthermore, the analysis and simulation show that the ICI power is determined by the product of Doppler spread and OFDM symbol period.