In underlay cognitive radio systems, secondary users (SUs) can share the spectrum with primary users (PUs) as long as the interference caused by SUs to PUs is below a certain threshold. When multiple SUs want to access the channel, multiuser diversity scheme can be applied to maximize the utilization rate of the channel. In this paper, the performance of selective multi user diversity scheme in underlay cognitive radio system is analyzed. We first start with the case in which all SUs have the same average signal-to-noise ratio (SNR). We then treat the more realistic case involving SUs with different average SNRs. Both power control and no power control scenarios are considered for the SUs. Furthermore, we take scheduling fairness into considerations and study a normalized SNR selection scheme in the case that SUs have different average SNRs.