Due to the popularity of wireless local area networks (WLANs), many research works for such networks have been done, mainly based on the assumption that the number of nodes, N, is known a priori and remains the same in the runtime. However, in the real world, N changes dynamically because wireless stations can be turned on/off or moved in/out anytime and anywhere. As a consequence, the theoretical results based on the assumption usually have their own limitations when considered with industrial applications in a WLAN. For this problem, we introduce in this paper a dynamic transmission control method to take into account the dynamic of WLANs. Based on the so-called p-persistent IEEE 802.11, the dynamic method can adaptively adjust the transmission probability in each back-off stage of DCF to achieve the maximum performance in the runtime. We use analysis to show how the method can achieve the maximal performance in theory. With simulations, we further confirm these results and show how the parameters involved can be settled to achieve the performance aims.