This paper analyzes the performance of cooperative decode-and-forward (DF) interleave-division multiple-access (IDMA) networks with best relay selection over Rayleigh/Weibull fading environments. In the analysis, approximate outage probability (OP) and bit error probability (BEP) expressions are derived for the considered system. Further, a 2-hop DF IDMA system is also designed to compare with cooperative one. We propose OP and BEP expression for 2-hop DF IDMA systems. Numerical results are obtained by changing the number of relays and the value of fading parameter in the proposed expressions. Also, simulations are provided to corroborate the exactness of the derived approximate OP and BEP expressions.