Abstract: Based on the micro mechanical feature of materials and the strength theory of fibrous bunchiness, the macro statistical size effect of materials was gained. As an example, the shear strength of specimens from a concrete project with three scales,i.e.,small, medium and big sizes widely adopted in soil strength tests, was discussed using the statistical analysis and the Monte-Carlo method. The result shows the statistical size effect of soil shear strength. The research indicates that different sizes of specimens result in different values of soil shear strength, and this phenomenon can be well explained using the strength theory of fibrous bunchiness. When the size of a soil specimen is bigger than medium size, its shear strength tends to be unchanged.