Firstly, the soil gravity force and seepage force are used to study the effect of seepage direction on the slope hydraulic gradient, safety factor and the excess pore water pressure existing on the failure plane. Four seepage directions including horizontal seepage, parallel to slope surface, vertical seepage and perpendicular to slope surface are considered. A series of related formulas is developed. It is found that the safety factor of horizontal seepage direction is lower due to the pore water pressure generated within the failure plane. In addition, a case study on the finite slope is analyzed to describe the importance of seepage direction. The initiated driving angle of debris flow is also affected significantly by seepage direction, especially when the seepage direction is near the slope angle. Finally, the results are compared with those found by Takahashi (1977, 1982).