Steady state stability of power system can be improved greatly by unified power flow controller（UPFC）. The power injection model and the real-time reliability model of UPFC are built. The installation cost model and the load curtailments cost model are then proposed. Based on the real-time reliability model， the load curtailments model with UPFC is developed to eliminate line flows and bus voltages violation with UPFC， so the probability of load curtailment is reduced and the reliability of power system is enhanced. With this model， a multi-UPFC allocation method is given to minimize the total cost of power system operation with proper reliability level. Simulation results of the RTS 79 system indicate that the model and allocation method are better than the traditional ones.