Abstract: In this paper, the principals of Orthogonal Frequency Division Multiplexing (OFDM) system was described and the effect of the Guard Interval (GI) on the system performance was studied. The role of the GI period to solve the problem of interferences that occur at the receiver was discussed. The optimum values of the GI parameter are determined, which were shown to have strong dependence on the maximum delay. A series of simulations have been carried out on Quadrature Phase Shift Keying (QPSK) modulation scheme over multi-path fading and Add White Gaussian Noise (AWGN).The relationship between Bit Error Rate (BER) performance and GI, Signal to Noise Ratio (SNR), the maximum delay spread and number of Sub-carrier was studied.