In recent years, VoIP over Wireless Local Area Networks (VoWLAN) technology has been increasingly on demand due to its advantages such as it is cheaper and supports mobility. Yet, the required stringent service quality for VoIP applications particularly in WLAN is still a challenge. This paper investigates the effect of two key parameters on VoIP quality; transmission/data rate and packet size. In this study, the data rate value can be 11, 5.5, 2, or 1 Mbps according to IEEE802.11b WLAN, while packet size is determined by the number of frames inserted in the packet. A simulation study has been implemented using OPNET Modeler. The first part of the results compares VoIP quality values obtained for each data rate. It also shows how decreasing the data rate degrades the VoIP performance. On the other hand, the second part of the results demonstrates how changing voice packet size can significantly affect VoIP quality. Further, it is discussed and analyzed the effect of packet size on delay, packet loss and network bandwidth requirement which accordingly affects VoIP quality. © 2012 IEEE.