With the advent of portable devices, power consumption becomes one of the most important considerations in VLSI designs. Multiplexer (MUX) is a basic component massively used in typical VLSI designs. In this paper, we focus on the minimization of switching activities in a MUX tree composed of 2-to-1 MUXes. The key contribution of this paper is: Given the on probabilities and the selection probabilities of input data signals, the proposed heuristic algorithm can properly encode all input data signals such that the power consumption of the resultant MUX tree is minimized. For a 64-to-1 MUX, the experimental results show that a MUX tree encoded by the proposed algorithm consumes 24% less power than a randomly-encoded tree on average.