The cooling and heating load reduction effect of an earth tube system under the climatic conditions of Korea was investigated using the newly developed earth tube model in EnergyPlus. Hourly, monthly and annual cooling and heating loads with and without earth tubes were compared and analyzed. It turns out that the earth tube has the larger reduction effect of the cooling load than that of the heating load. When properly designed and predicted, earth tubes can reduce significant amounts of cooling loads. Finally, the higher earth tube airflows do not necessarily guarantee the larger cooling load reduction due to the higher discharge air temperature entering the room.