Frequency selective channel distortions in the Multi Carrier Code Division Multiple Access (MC-CDMA) system is reduced with the use of guard interval. In previous works, guard interval is associated with root mean square delay spread of the channel (τrms) and the symbol duration. This paper investigates the sensitivity of MC-CDMA performance to guard interval and demonstrates a novel approach to determine the Optimum Guard Interval (OGI). System performance is determined from the Signal to Noise Ratio (SNR) level of a detected bit. The OGI for a given channel profile and system parameters is determined as the guard interval with maximum SNR. Unlike the previous studies, the OGI was investigated for different Es/N0. The OGI are growing increasingly with Es/N0 and is equal to maximum delay spread for Es/N0=30dB. We show that the OGI used depending on Es/N0 can be selected as the delay window that includes certain multipath power. With such a method with different frequency selective channel profile the optimum guard interval without the need for complex operations can be identified.