Cooperative diversity provides spatial diversity gain via virtual antenna array and thus improves the communication capacity in fading channels. Although space time code (STC) design for continuous phase modulation (CPM) is a new research topic, recent papers illustrated a number of advantages relative to linear modulations such as PSK, QAM etc. In this paper a cooperative diversity system with orthogonal ST coded minimum shift keying (MSK) is introduced. The relay terminal operates both amplify and forward (AF) and decode and forward (DF) modes. Simulation results show that proposed system is very convenient for fading channels since it provides diversity gain and utilizes advantages of continuous phase modulation.