Abstract: In terms of standards recommended by 3GPP and ETSI, the performance of voice activity detection (VAD) algorithm applied for adaptive multi rate(AMR) speech codec was simulated and analyzed with varying background noise and different signal to noise ratios. Based on a great deal of test results, the robustness of AMR VAD applied for 3G system was verified and compared with ITU G.729 VAD. It proves that the AMR VAD performs better than G.729 VAD in nonstationary background noise environment, the capacity of mobile communication system is expanded about 30% when the intelligibility and naturalness of reconstructed speech is not impaired.