Abstarct: Cognitive radio is an approach in order to enhance efficiency of using spectrum. The basis of this approach is to divide the spectrum between primary and secondary users. The primary users are allowed to use the intended spectrum and can access to it any time. The secondary users access to the spectrum dynamically and opportunistic, provided to not interfere unbearably with primary users. In the present thesis we examined the spectrum leasing in cognitive radio grate baxsed on OFDMA (Orthogonal Frequency Division Multiplexing Access) for keeping secret forwarding information of the primary grate. In this grate, the secondary users serve as decode and forward relay stations and help the primary grate in forwarding information. In this grate, it is assumed that the band width divides into n sub-carrier. The primary grate is an OFDM and the primary users can simultaneously use more and one sub-carrier, while the secondary users can only forward information on a single sub-carrier. Moreover, there are some .... users in the intended grate who attempt to eavesdrop the forwarded information. In that grate, forwarding information is made as frxame to frxame, in which each frxame includes two periods. In the first period the primary baxse station forwards information, while secondary users listen to primary baxse station. In the second period, secondary users who have selected to help forwarding information of primary grate, encode received information from primary grate, re-decoding them, and forward the decoded information to the primary user. Other secondary users, if there are, forward their information to the secondary baxse station in the second period. It can assume various scenarios for eavesdropper, like that the eavesdropper eavesdrops forwarded information from primary baxse station in the first period, or it eavesdrops forwarded information form secondary users in second period. The purpose of this study was to assign suggested sources, select proper secondary users to help forwarding information of primary grate, assign sub-carriers to secondary users, assign power of primary baxse station and secondary users, such that guaranteed the secrecy of forwarding information of primary grate and maximize the secrecy rate of secondary grate information forward. We used dual Lagrange and sub-gradient methods for solving problem.