In this paper, we propose a stochastic rate control method to provide seamless video streaming for vertical handoff between WLAN and 3G cellular network. In the proposed method, we first estimate the channel rate by using the state transition probabilities that can be found from the relationship between the packet loss ratio (PLR) and the medium access control (MAC) layer parameters. The proposed method performs bit allocation at the frame level using the estimated channel rate, minimizing the average distortion over an entire sequence as well as variations in distortion between frames. Experimental results indicate that the proposed method provides better visual quality than the existing TMN8 rate control method in heterogeneous wireless network.