Two phase flow mal-distribution in inlet header of the parallel flow evaporator will cause performance degradation, partial frosting and comfortableness problems. In order to solve these issues in heat pump system of electric vehicles, four types of small diameter tube and fin heat exchangers with different flow passage were designed and experimental measured in heat pump system of electric vehicles. The experimental results showed that in terms of performance, the small diameter tube and fin heat exchanger can reach even exceed the micro-channel heat exchanger on capacity and COP in heating model. Compared with micro-channel, the tube and fin heat exchanger with 4 inlets and 4 outlets can increase capacity from 2010W to 2689W, and increase COP from 2.6 to 2.8.However the frost/defrost experimental results showed that there was a decrease on the capacity of micro-channel heat exchanger after several frost/defrost periods. For the small diameter tube and fin heat exchangers, the condensate water was easier to be drained, thus partial pressure difference and heat exchange capacity would recover to the initial value eventually. Therefore small diameter tube and fin heat exchangers possess a good potential in the application used in heat pump of electric vehicles.