Abstract: To investigate the critical hydraulic condition of contact erosion between cohesionless soil layers under horizontal seepage flow, two instability modes of removable soil particles are established at the meso-level. A force analysis based on the sliding and rolling process is employed for a single removable fine-grained soil particle at the interface between the coarse- and fine-grained layers, while the laminar flow hypothesis is used to analyze the flow shear stress. A theoretical formula for the critical hydraulic gradient with overlying coarse-grained soil and underlying fine-grained soil is proposed. This formula is then simplified by considering the characteristics of the location where the fine particles initiate in the experimental observation. It is verified by comparing the calculation with existing empirical formulas according to the experimental data. The results show that the particle size ratio of the coarse- and fine-grained soil and the particle size of the removable fine-grained soil are the two primary factors of the critical hydraulic gradient. The influence of porosity of the coarse-grained soil is not apparent. The critical hydraulic gradient is negatively correlated with the particle size ratio and porosity of the coarse-grained soil, while it is positively correlated with the size of the removable fine-grained soil particle. When the particle size ratio is no greater than 10, contact erosion does not occur. When the particle size ratio is greater than 10 but is no greater than 50, the critical hydraulic gradient is mainly affected by the particle size ratio and the size of the removable fine-grained soil particle. When the particle size ratio is greater than 50, the main factor becomes the size of the removable fine-grained soil particle. The theoretical formula considers the distribution of the fine-grained soil particles that determines the value of d_i, which is recommended not to exceed d₅₀. When the distribution characteristics are not available, d₁₅ is recommended as the value of the removable fine-grained soil particle size.