Abstract: Cooperative diversity technology can be utilized to upgrade the transmission performance of system remarkably in the wireless network. Idle spectrum hole can be more fully utilized through joint temporal and spatial sensing in the cognitive network. In this paper, the outage probability performance of the cognitive relay network employing joint temporal and spatial sensing where the selection decode-and-forward (SDF) cooperative transmission protocol is utilized is analyzed. The exact and closed outage probability expression is obtained in Rayleigh fading environment. It is shown from analysis and simulation results that the outage probability performance while employing joint temporal and spatial sensing is better than pure temporal sensing and pure spatial sensing, moreover, the transmission performance of cognitive user is improved greatly while introducing cooperative diversity technology. It is because the derived theoretical results on outage probability can be utilized to measure and evaluate the performance of spectrum sensing algorithms and cooperative transmission schemes in the cognitive network that they have important theoretical and practical values.