The performance of multiuser diversity in cooperative relay networks is analyzed. Closed-form expressions of outage probability and symbol error probability for decode-and-forward (DF) based cooperative relay networks are derived. The deduction is validated by simulations which show that a (M+1)K-fold diversity gain can be achieved when adopting multiuser diversity based scheduling (where M is the number of available relay users and K is the number of accessing users). These results demonstrate that, in cooperative relay networks, both relay users and accessing users are source of the multiuser diversity gain.