The experiments were conducted on 5 mm and 7 mm finned tube heat exchangers with same refrigerating capacity based on enthalpy method. The evaporator simulation model was established to analysis the influence of tube diameter on the heat transfer performance. The results show: 1) the heat transfer coefficient on the air side of the 5 mm tube heat exchanger is 17% higher. 2) With same refrigerating capacity

the refrigerant mass flow rate of 5 mm diameter tubes has a 4.6% reduction and its mass velocity increases by 89.4%

which arises the dry out point in advance. 3) At same vapor quality points

its local heat transfer coefficient increases to 1.43 ~ 1.86 times as that of 7 mm diameter tubes. Meanwhile friction pressure drops

acceleration pressure drops and local pressure drops in 5 mm diameter tubes increase to more than 3 times as those in 7 mm diameter tubes

arising 1.1 ℃ increasing in evaporating temperature.