In this paper, using of the spherical ground storage tank as a heat sink for cooling of the residential and office buildings in the climatic condition of Tabriz city is studied. For computing the heat transferred between the underground storage tank and the soil around it a new analytical solution is presented. Inside the storage tank, the analogy between thermal and electrical conduction is used for deriving heat balance equations. In ground around the storage tank, the transient conduction heat transfer equation in spherical coordinates is considered. The Sample building envelope is simulated in EnergyPlus and the storage tank and fan coil is simulated in Matlab software. Finally the building and the underground storage tank is linked together and the performance of the direct cooling system is investigated. Results show that the dimension of the storage tank has significant effect on the comfort condition of the building. For a building there is an optimum diameter that thermally discomfort hours is minimum. With decreasing the storage tank optimum diameter the discomfort hours increases significantly. Using storage tank with larger diameters than optimum diameter the discomfort hours increases very slightly. Also results shows that the optimum diameter for office building is lower than the residential building.