This study addresses detection of primary users (PUs) for cognitive radio system (CRS) with distributed spectrum sensors (DSSs) in Rayleigh fading environment. Two sensing schemes using DSSs that employ energy detection (ED) are investigated as cooperative sensing and collaborative sensing. For the cooperative sensing, each DSS makes hard decision after sensing and transmits the information by on-off keying (OOK) singling to a secondary user (SU) that performs data fusion. In collaborative sensing DSSs exchange the hard decision information after sensing and make data fusion at a selected DSS. Then, the selected DSS transmits the decision information to the SU with OOK signaling. In this paper, a closed-form expression for the probability of detection (PD) of ED with selective combining (SC) in Rayleigh fading environment is derived. Applying this expression to the PU detection problem, analytical models for evaluating the two sensing schemes are obtained. Results show that at 5-dB signal-to- noise ratio (SNR) and using 3 DSSs, the PD is increased from 0.02 to 0.3 and 0.4 by collaborative sensing and collaborative sensing, respectively. © 2009 IEEE.