The transmission of multimedia data over WLAN is going to be an unavoidable trend. When packet is transmitted over wireless network, small packet length will increase the header overhead and long packet length will tend to increase the packet error rate. In this article, an adaptive video transmission mechanism under WLAN is proposed. The optimal packet length can be obtained through mathematical analysis according to the factors such as current wireless network transmission status and the header overhead. Based on the optimal packet length, layering and scalability characteristics of MPEG-4 FGS and unequal error protection, a video packet transmission mechanism of low calculation complication is thus proposed. Several distinct scalable video transmission schemes over burst-error channel have been compared, and the simulation results reveal that our transmission scheme is capable of reacting to the varying channel condition with less and smoother quality degradation.