Abstract: The suffusion involves selective erosion and gradual migration of fine particles through the voids of soil skeleton formed by coarse particles under seepage flow. As a result, redistribution of soil skeleton stress and deformation of soil may be induced. In this study, a series of suffusion tests are carried out using the triaxial erosion apparatus with measurable local pore pressure. The effects of the initial fine particle content and initial relative density on the suffusion of a gap-graded cohesionless soil are investigated. According to the spatial-temporal evolution of local hydraulic gradients along seepage path, the evolution process of the suffusion is revealed. Test results show that both the initiation and the failure hydraulic gradients of the gap-graded cohesionless soil increase with the increase of the fine particle content and relative density. The cumulative loss of fine particles decreases significantly with the increase of the relative density. When the relative density increases to a certain value under isotropic stress condition, the gap-graded cohesionless soil will change from an unstable state of seepage to a stable one. Additionally, the internal manifestation of suffusion initiation of soil is the mutation and uneven distribution of local hydraulic gradient along seepage path. The suffusion will cause the loss of fine particles, as well as the increase of the void ratio. Under the isotropic stress condition, the volume shrinkage is induced.