This letter derives a simple and uniform closed-form expression for the asymptotic outage probability of the decode-and-forward relaying transmission in generalized fading channels, including Rayleigh, Ricean, Nakagami-q, and Nakagami-m fading. Various numerical results show that the derived uniform outage probabilities are tight approximation in the high SNR regime. The closed-form expression is helpful to investigate the outage probability of cooperative networks under different fading conditions.