Installing small wind turbines on the roof of buildings is a new way of wind power utilization in recently years. Based on the annual mean wind speed, the efficiency of wind power utilization on roofs of buildings was studied and the evaluation indexes for the utilization efficiency were proposed. With the help of CFD numerical analysis, the analysis on wind power utilization efficiency on flat roofs of rectangular buildings was carried out. The performance indexes of reference points under different windward angles were investigated. The best position and arrangement of wind turbines on the roof were also discussed. Furthermore, the utilization efficiency of four roofs with different shapes were analyzed and compared with that of flat roof.