This paper investigates the utility-based resource allocation for cooperative spectral sharing in orthogonal frequency division multiplexing (OFDM)-based cognitive radio (CR) system. We study the multicarrier CR system with a primary user (PU) and multiple secondary users (SUs). By applying a cooperative spectrum leasing protocol, SUs can serve as amplify-and-forward (AF) relays to assist PU's transmission. Taking both the fairness and the throughput of SUs into account, we aim to maximize the total utility functions of SUs and formulate a joint optimization problem under constraints of the target rate of the primary system and the total power of the secondary system. The resource allocation involves rate arrangement, power control, subcarrier-relay selection, subcarrier allocation and subcarrier pairing. The proposed resource allocation policy is obtained by using the Lagrange dual approach. Simulation results demonstrate that the proposed scheme can make balance between throughput and fairness among SUs.