The effect of the opportunistic spectrum access on the spectrum utilization is studied in this paper in terms of the secondary network capacity measured at the secondary receiver. A mathematical model is developed to represent the secondary network capacity in Rayleigh fading channel. An exact analytical solution for the capacity is derived for both sensing and accessing fading channels. A numerical evaluation of the channel capacity is presented for different channel sensing and accessing schemes. The effects of detection and accessing channels' parameters on the capacity are investigated. Numerical results showed how the utilization of the network can be increased significantly by the suggested opportunistic spectrum accessing technique. It was found that a good sensing system with a high secondary user signal to noise ratio accessing the licensed band increases and drives the spectrum utilization to its maximum. The capacity can be increased by three factors, secondary accessing channel, primary user interference or the desired primary user's QoS. The awareness of proper sensing scheme will maximize the spectrum utilization without degrading the QoS of primary users.