Abstract: Researches on strength of soft clay have an important significance on the evaluation of engineering property and stability analysis of soil slopes.The relationship between micro-structural parameters of fracture surface and strength of soil,including cohesion and internal friction angles,was discussed when the the peak strength was reached and the soil was faced with failure.Choosing the representative marine soft clay samples in their natural states in the middle area of Pearl River Delta conducting the physical and mechanical tests,a series of data were obtained.Scanning the micro-structure of soils taken from the failure surface when the soil samples reach failure state in the shear test(including direct shear test and quick tri-axial shear test),the micro-structural images were gained.Then,the relationship between micro-structural parameters and strength coefficients(c and ?) was established by image-processing technique.The relationship between macro-parameters including plastic index(IP) and clay grain content(Pc) which could well reflect the micro-structural features was discussed.The results showed that the shear strengths of soft soils were coincident very well with the variation of micro-structural parameters.These results could be used in the stability control of high fill engineering etc.