[[abstract]]Due to the rapid development of the network technology , and the improvement of digital signal processing technology, it is a more and moreobvious trend that traditional public switched telephone network is replaced byIP network Recently, VoIP has become an inevitable trend, numerous servicesproviders of VoIP all over the world are now devoting themselves to the highly competitive market. Service quality (QoS) of VoIP is the focal point for eachservices provider to make efforts on , it is also one of the key factors for usersto consider when choosing the products. In this study, emulation approach is adopted in experimental part. Firstly, an experimental environment is set up in order to measure QoS, it includes anetwork emulator and a VoIP network platform which is consisted of SIP server and Softphones. This study attempts to integrate VoIP network platform and the network emulator for emulative tests of different conditions. The networkemulator is used to create and emulate various impacts in WAN environment, such as: packet delay , packet loss , jitter etc. In addition , when calls are made within a VoIP network platform , an uncompress-standard Codec (G.711) is adopted, to measure voice quality andcompare the influence on voice quality (MOS value) during the conversation.With the analysis and discussion of the results derived, it is expected to havebetter understanding of the influence that these parameters may have on theapplication of the VoIP. The test results can be offered to the engineers to study and decide the states to be paid special attention to while designing anddisposing an proprietary VoIP network and scheme and offer the suggestions ofimprovement to the network designers or users.