Abstract: The difference of shearing strength resulting from the direct shearing test and the tri-axial compression test and factors contributing to such a difference are seldom discussed. This paper addresses this issue by evaluating the effective shearing strengths of 11 sets of samples from 3 trial pits in 2 slopes of Hong Kong, which are obtained by the tri-axial test and direct shearing test. The results indicate that the group kt consisting of mid-fine particles and a smaller porosity has higher shear strength than the group skm consisting of mid-coarse particles and a larger porosity. For the same group of samples, the shearing strength obtained from the tri-axial test is larger than that from direct shearing test. The difference in effective cohesions c' ranges from 60 Pa to 60 kPa,while the difference in the effective friction angle ' ranges from 2 to 18. The effective cohesions (c') is linearly correlative to each other with a coefficient of correlation (R) of 0.849,while the effective friction angles (') is logarithmically correlative to each other with the R of 0.686. In addition, the differences of c' have a reciprocally linear correlation with the differences of ' and with the R of 0.377. The difference in shear strengths depends on material components and microstructure features, e.g. clay particles, quartz grains and clay minerals content. Among them, the clay content is logarithmically correlated with the difference of c and with the R of 0.776,while it has a reciprocally logarithmic correlation with the difference of ' and with the R of 0.733. Therefore, it is important to pay special attentions on variations in the clay content and the quartz content. This paper also discussed other factors that should also be considered, including the natural density and free iron oxides ratio.