An in-situ thermal response test is applied to the thermal performance evaluation of the vertical closed U-loop ground heat exchangers (GHX). A line source method is used to derive the thermal conductivity and borehole thermal resistance from data obtained by the thermal response test. Tests are made for four types of GHXs with different borehole configurations. Results are presented in terms of configuration and operating parameters of GHXs to investigate their effects on the thermal performances. Results show that while thermal conductivity moderately increases with the increase of heat transfer area either by the use of larger diameter of tube or insertion of more tubes, the borehole thermal resistance and the temperature entering the ground source heat pump (GSHP) are considerably reduced. Of the borehole thermal resistance components evaluated, the grout thermal resistance is the most governing one in the borehole heat transfer (more than 65% of the total borehole resistance), while convective thermal resistance in the tube is almost negligible (less than 2.5%). (C) 2015 Elsevier Ltd. All rights reserved.