When a difference of specific weights of supplied and purged gas exists in an oil tank of a tanker, the gas flow and gas dispersion during gas purging are varied with discrepant values of each specific weight. In case of the heavy gas being purged by the light gas, the interception of gas purging is especially remarkable by the existence of stagnated flow. In this paper, two dimensional analyses of flow velocity and gas concentration in tank models by finite element method are carried out to make clear the relations between a difference of specific weights of both gases and efficiency of gas purging. The effects of the location of inlet and the arrangement of inner structures of tank against gas flow and gas dispersion are also clarified due to discrepancy of specific weights of gases. In case of forming the interface between supplied and purged gases, the criterion of arising this phenomenon and the governing equation of gas concentration in tank as the time proceeds are presented. Comparing the calculated values of gas concentration and the measured values in actual oil tank, good agreements between them are shown.