Abstract: A mathematical model for computing the out-of-band (OOB) leakage radiation in NC-OFDM systems and the interference to the adjacent primary users (PUs) is presented in this paper. Based on this mathematical model, the optimal parameters could be obtained by solving the nonlinear programming (NLP) problem subject to secondary users(SUs)' spectrum overhead and PUs' occupancy rate <em<r</em<. The results showed the close relationship between the number of guard subcarriers and the parameters of input back off (IBO) as well as PU occupancy <em<r</em< and interference temperature tolerance. The simulation results have shown that, under the conditions of given PU occupancy <em<r</em< and PU interference temperature limit, the solution of proposed programming problem can be used to design the optimal system parameters for secondary users with the smallest number of guard subcarriers and the demand of system to the parameters of IBO.