Abstract U-type evacuated glass tubular solar collectors are increasingly used for solar-assisted heat pump system for space heating, and it is an ideal choice as an evaporator for this system. The temperature of the working fluid in such kind of collectors is commonly lower than that of the ambient air. However, it was rarely reported in literatures. In this paper, the thermal performance of the U-type evacuated glass tubular solar collector is studied experimentally under conditions where the temperature of the working fluid is lower than that of ambient air. The collector which consists of 16 U-type evacuated glass tubes is tested. Thermal efficiencies of such solar collector are obtained for the typical conditions with the solar irradiance of 375 W/m 2 , 435 W/m 2 , 535 W/m 2 , 675 W/m 2 and 735 W/m 2 generated by a solar simulator, and the mass flow rate of 0.02 kg/s, 0.04 kg/s, 0.06 kg/s, 0.08 kg/s, 0.1 kg/s. The results suggest that the thermal efficiency is higher at negative reduced temperature and it is positively correlated with mass flow rates and solar irradiances, as well as negatively correlated with inlet temperatures. Additionally, taking into account the error effects, the lower solar irradiance is, the higher sensitivity to changes of the mass flow rate and the solar irradiance efficiency changes will have. The results could be a guideline for the application of U-type evacuated glass tubular solar collector in solar-assisted heat pump system for space heating.