For determination of the ground horizontal resistance coefficient, m, of a pile under lateral loading, we usually assume horizontal surface for natural soil foundations. However, there has been no relevant standards or experiences for evaluating the coefficient m for inclined ground. In order to evaluate the coefficient of ground horizontal resistance of gravel soil on slopes, static lateral loading tests are conducted in laboratory to examine the horizontal bearing behaviors of a pile embedded in the gravel soil slope. The variation of m value of the subgrade with slope gradient is discussed. A formula modified using m value for horizontal gravel soil ground is used to determine the m value of gravel soil foundation on slopes with different gradients. The suggested range of m value is provided in the procedure. The applicability is verified by field test. It is shown that the m value ranges from 23 to 65 MN/m4, and decreases as the gradient of slope increases.