Abstract: In order to study the mechanics characteristic of silt using in highway embankment in eastern Henan Province, the unsaturated direct shear test was conducted using unsaturated direct shear apparatus with controlling suction. The contribution of the suction to the unsaturated strength for silt was mainly analyzed. The effects of dry density to strength were also discussed. The results showed that the particle size of eastern Yellow River alluvial silt was concentrated on 0.005~0.075 mm. The compaction curve appeared asymmetrical "double hump" phenomenon, but the peak values were different obviously. Attention should be given to the accuracy of the maximum dry density in construction. Dry density and suction had different influences on the cohesive force c. The cohesive force changed with the decrease of the suction, which was like parabola. The maximum cohesive force appeared near the optimum moisture content. The cohesive force increased with the increasing of dry density. The effect of suction to the angle of internal friction was small within the range of 5 °. In general, with the increase of dry density, the angle of internal friction increased slightly. For the silt, the effective angle of shearing resistance (φ') and the angle of shearing resistance with respect to matric suction (φ^b) was not constant, where φ' was influenced by the matric suction and φ^b was influenced by matric suction and vertical stress together. That is to say, the Mohr-Coulomb failure surface was bidirectional bending in the space of τ_f-(u_a-u_w)-(σ_n-u_a).