The Voice over Internet Protocol (VoIP) market is booming because of its economical benefits and innovative phone services. While the popularity of VoIP has already closely approached that of the Public Switched Telephone Network, VoIP's quality of service still cannot match that of traditional phones. This is because of distinct characteristics ofVoIP, namely real-time and playback, and limitation of the Best-Effort service. The solution provided by the Internet Engineering Task Force to the quality issue is Differentiated Services (DiffServ). This thesis evaluates the performance ofVoIP in Best-Effort network services and in DiffServ network services. Best-Effort services and DiffServ network services were created in a simulation environment with the OPNET Modeler. VoIP's performance was observed, while varying the load of background network traffic and VoIP network traffic. The experimental results show that VoIP applications can maintain good performance in DiffServ Telephony network service yielding low end-to-end delay, low jitter, low packet loss, regardless of traffic conditions, but cannot do so in Best-Effort service.