Abstract: In order to reflect the actual channel attenuation, a model selection algorithm for path loss in wireless sensor networks is proposed. Firstly, the statistical properties of some path loss models are analyzed,and then Expectation Maximization (EM) algorithm from incomplete data of Received Signal Strength (RSS) is proposed for parameter estimation. Finally, a set of weighted coefficients are given on the basis of criterion function, which could select an appropriate path loss model. Through experiment, the proposed model selection method could estimate parameters effectively, compared with other similar algorithms,this method could pick up a model fitting the experimental data better.