Internal erosion occurs when fine particles escape from the soil driven by seepage flow, which is considered to be the crucial factor causing the failure of earth structures filled with gravelly soil. The objective of this paper is to suggest an appropriate method to assess internal erosion potential of gravelly soil. By analyzing the sensitivity of soil material to internal erosion, the variable (D-c15/d(f85))(max) and the content of coarse particles (P-c) are selected as the evaluation indexes (D-c15 and d(f85) are the diameters of 15% mass passing in the coarse component and 85% mass passing in the fine component, respectively). A series of gravelly soils with different particle size distributions are tested for internal erosion by the self-made permeameter. Based on the test results, an evaluation method for the internal erosion of gravelly soil is proposed. Gravelly soil is prone to internal erosion when 60% <= P-c < 95% and (D-c15/d(f85))(max) > 9.5. The proposed method shows good accuracy in evaluating the internal erosion of 36 soil samples from other studies, which confirms the reliability of the method. The proposed method makes it possible to accurately assess internal erosion of gravelly soil, and an alternative method is provided for engineers to determine whether there is a risk of internal erosion in earth structures consisting of gravelly soil.