Voice over Internet Protocol (VoIP) is an application from the internet services that allows the user to communicate with each other. VoIP requires Quality of Service (QoS) because it is very sensitive to loss and delay of the information. Therefore, QoS of VoIP is needed to ensure that voice packets are not delayed or lost during the transmission over the network. This paper is to study the effect some of the security mechanisms on the VoIP QoS. In this paper, simulation tool OPNET modeler version 14.0 is used to implement the proposed network (VoIP Network).