In this work, we will focus on which modulation scheme should be supported given the knowledge of SNRs of all links in a simple DF cooperative communication system where only one relay is present. Two rules are proposed, namely the exact rule and the approximate rules. The exact rule is derived based on the exact SER. However, the decision of which modulation scheme in the exact rule is used for current transmission is not straightforward. This motivates us to derived the approximate rule based on three key observations. The conducted simulation shows that the performance of the exact rule and the proposed approximated rule are similar in terms of average rate.