A simple one-dimensional ensemble average PBL model, including condensation, evaporation, atmospheric radiation, and the surface energy budget, is used to study the evolution of the planetary boundary layer in a cloud-free atmosphere. In this model, the turbulent kinetic energy E is predicted by a prognostic equation. The length scale (l) of Sun and Ogura, and Deardorff is modified; the eddy coefficient is proportional to El, as suggested by Deardorff; and the similary equations proposed by Businger et al. are used for the surface layer. In addition, the force restoring method is adopted to predict the surface temperature and an analogous method is used to calculate the surface temperature and an analogous method is used to calculate the surface soil moisture. The model is used to simulate the data of Wangara Day 33. The simulated results are in good agreement with both those observed and those produced by the other more complicated model.