The limitation of the random walk theory of energy transfer between rare earth ions in crystals published in literatures is pointed out in this paper. A modified model, which can be used in any level of rare earth ion concentration when the interaction between donor and acceptor is different from that between donors, is introduced. This model has been applied to analyze the measured fluorescence lifetimes of Yb3+:YAl3(BO3)4 with a series of Yb3+ concentrations.