Cyclic Prefix (CP) is one of the key OFDM parameters. It is used to completely eliminate both Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI) as long as the CP length is greater than the channel delay spread. By eliminating the ISI and ICI, the CP compensates for the effect of the multi-path dispersion; but it consumes a considerable amount of the scarce spectrum and the power. Contemporary OFDM systems usually use a fixed and large CP length to tolerate worst case channel condition. This technique, however, causes a loss in bandwidth efficiency as well as consumes relatively more transmitter energy. Therefore, there is a need to adopt the CP length based on the channel parameters. This paper investigates the effect of varying the CP length on the OFDM system over different wireless channel models, where the variable CP length is estimated based on the Maximum Excess Delay Spread (MEDS) of the channel. According to this method, the estimated CP length optimizes the system capacity and improves the overall system performance.