Video transmission is one of the popular research topics of the multimedia communication. Since various channels have different characteristics, video transcoder can be used to adapt a pre-coded video bitstream to satisfy these channels. A problem faced by the transcoding systems is the way to decide the optimal frame rate for different channels. In this paper, this problem is converted into the constrained optimization problem, and the frame rate decision method is presented based on Lagrange function. Experimental results show that the proposed method can achieve higher coding performance than the conventional algorithm.