There is a growing demand for the transmission of multimedia applications over wireless computer networks. The aim of this study was to analyse the effects of minimum Contention Window (CWmin) on Quality of Service in IEEE 802.11 MAC protocol. This study was carried out using ns-2 simulation software. The analysis was based on four Variable Bit Rate (VBR) connections with different packet sizes, distributed randomly in the same Basic Service Set (BSS). It was demonstrated that poorly selected values of CWmin result in considerable network performance degradations. Moreover, the CWmin value for achieving optimum QoS proved to be different. Based on this work, an intelligent method for determining the CWmin size to achieve optimum QoS has been proposed.