Cooperative diversity is one of the promising solutions to overcome the channel impairments and provide high data rate coverage. It has been studied for different channel fading as Rayleigh, Rice and Nakagami-m. Nevertheless, for a decode and forward relaying over composite fading including simultaneously multipath and shadowing, outage results have ever appeared. Using the Nakagami probability density function (pdf) to model multipath fading and the Gamma pdf to model shadowing has led to the generalized-K model for composite fading. This paper gives an analytical expression of the outage probability of the Decode and Forward cooperative technique over the generalized-K fading channel. Simulation results are in good agreement with our theoretical analysis.