In cognitive radio spectrum sharing system, the secondary user transmitter occupies the spectrum licensed to primary user, when primary user does not occupy it. in this proposed approach of the spectrum sharing communication system, allocating the subcarrier randomly to secondary user in the absence of primary user’s channel information with certain probability derives the performance of the ofdm based cognitive radio network. to avoid the interference and to maintain qos of primary user, the secondary user transmitting power is controlled below the predefined threshold refers to as interference temperature. the average capacity of secondary user with sub carrier collisions is employed as performance measure to investigate the proposed random allocation scheme for both general and rayleigh fading channel model. to obtain the maximum sum rate of secondary user an efficient centralized sequential algorithm is proposed, that assigns randomly selected sub carrier to secondary user to maintain orthogonality among those sets.