This study deals with the thermal performance of horizontal earth coil heat exchangers. There are four types of earth coils which were made of polyethylene pipes. Each type of them was buried into the depth from 1.0m to 1.8m and the length of 5.0m, respectively. Three types of them are the spiral coils and their diameters are 0.8m. The pitch of two spiral coils is 0.2m. And pitch of the third one is 0.4m. The last type of the earth coil used in the experiment is consisted of horizontal laid pipes in six rows. Experiments on the heat extraction from the soil utilizing four types of coils have been continued for two years. During first year of the experiment, the heat extraction rates of each type of coils were examined. Results showed that there was not so big difference of heat extraction rate among those four types which were buried into the same size of soil. The capability of the seasonal heat extraction utilizing the horizontal coil was confirmed. The soil temperatures around the earth coils were recovered to the natural temperature of the soil till the beginning of next winter. During the second year, the experiment of the heating, utilizing heat pump system connected to the earth coils, was conducted. The heating load, used for the experiment, was calculated by HASP code as a demand of the typical one family house. The result of the average temperature of the working fluid in the coil was -1.6℃ during five months experimental period. Although, the average temperature of heat extraction was lower than the average outdoor temperature in winter, a higher C.O.P. value of the ground-coupled heat pump system, compared to the air source heat pump system, has been obtained. The average C.O.P. of the present system in the winter was 2.38 constantly during the experiment.