We propose and evaluate a cooperative system based on bit-interleaved space-time coded modulation (BI-STCM) with the iterative decoding, referred as CO-BISTCM system. We present an analytical approach to evaluate the performance over frequency flat-fading. The channel coefficients are modeled as Nakagami-m distributed, which covers a wide range of multi-path fading distribution for different values of m-fading parameter. Our analysis is focused on space-time coded cooperative (STC-CO) and orthogonal space-time block coded cooperative (OSTBC-CO) protocols for the transmission through both fixed-gain amplify-and-forward (AF) and decode-and-forward (DF) user cooperation with maximal ratio combining (MRC) at the destination. The closed-form moment-generating function (MGF) of the branch metric is derived. We derive an expression for bit-error rate (BER) of CO-BISTCM system using M-ary phase shift keying (M-PSK). The derived expression also shows the diversity order of the system. Finally, theoretical and simulation results are shown and they seem to agree well.