In this paper, we analyze the performance of cooperative diversity using adaptive Decode and Forward (DF) relaying over independent non identical flat composite fading channels which include multipath fading and shadowing simultaneously. We have considered the generalized-K model which is very versatile and accurately approximates many of the commonly used channel models as Nakagami-m, log-normal distribution and the mixture of the two distributions for the composite fading. We give an analytical expression for the symbol error probability of the proposed scenario and analyze its dependence on the channel parameters. We assume in our context, that the relay decides independently whether or not to forward the signal to the destination based on the received signal quality. Simulation results are in good agreement with our theoretical analysis.