Only one-third of vehicle fuel heat energy is used to drive the motor,while the remaining two-thirds directly exhaust through the air and cooling water.It is not only a huge of waste but also pollutes the environment.So the waste heat of motor was studied as a heat resource of organic Rankine cycle(ORC). First a mathematical modeling was constructed based on ORC system,then operating condition was changed to evaluate different work fluids.The results showed R245fa and R123 are the proper work fluids for ORC.Compared with other researchers’data,the model was correct.Thus the study is the preliminary design of ORC system.