This paper presents the pullout behavior of suction pile using finite difference method; and the commercial software, FLAC3D, was employed for the numerical analyses. The conventional equation for predicting ultimate pullout capacity for sliding failure was compared with the numerical analysis results. The pile diameter, pile length, and the undrained shear strength of the clay were varied to investigate the effect of the parameters. Based on 3-D analyses of total 32cases, it was concluded that the conventional equation for sliding failure could be adopted reasonably if the interface coefficient is reasonably provided. The interface coefficients were found to vary depending on the undrained shear strength, pile length-diameter ratio. This paper presents interface coefficients which could be used to the internal and external side friction of suction pile.r sliding failure was compared with the numerical analysis results. The pile diameter, pile length, and the undrained shear strength of the clay were varied to investigate the effect of the parameters. Based on 3-D analyses of total 32cases, it was concluded that the conventional equation for sliding failure could be adopted reasonably if the interface coefficient is reasonably provided. The interface coefficients were found to vary depending on the undrained shear strength, pile length-diameter ratio. This paper presents interface coefficients which could be used to the internal and external side friction of suction pile.