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Cooperative communication system has emerged as a significant concept which improves reliability and throughput in wireless communication systems. Some existing cooperative diversity protocols have been shown to increase the diversity order, allowing single-antenna nodes to cooperate with each other and achieve high performance like a real MIMO system. In this paper, we focus on improving the precision of analysis of Amplify-and-Forward (AF), Decode-and-Forward (DF) and Hybrid Amplifyor-Decode and Forward (HADF) in terms of outage probability. Especially, we derive some approximate expressions of outage probability for these three relaying protocols. The approximate expressions are proved to be better than some existing ones, in other words our approximations are closer to the truth value, especially when Signal to Noise Ratios (SNR) is very low. In our research, we use the outage probabilities calculated by Monte Carlo Method (MCM) as the reference for the reason that they are the nearest to the truth value. We also show analytically and experimentally that our expressions are better than some existing approximate expressions in low SNR. We can prove the accuracy of the expressions through the analysis of simulation results. Besides, the approximate expressions can be reused when we investigate the outage probability of cooperative diversity protocols.