The distribution of rock masses discontinuities is random and irregular; but in the statistical sense

the property of rock masses discontinuities distribution is self-similar. The three-dimensional network numerical modelling technique is used to establish the model of randomly distributed discontinuities in the statistical homogeneity of structure; the structure of jointed rock masses is regenerated on the screen of computer. Then the fractal dimension of the rock masses discontinuities distribution is computed applying fractals. The distribution of rock masses discontinuities has scale effects; and the fractal dimension of the rock masses structure decreases with the size of the rock masses specimen increasing; but when the specimen dimension increases to a certain value，the fractal dimension varies decreases no more. This certain fractal value is named "representative fractal dimension"

and the value can be used for describing the distribution of discontinuities. By comparing "representative fractal dimension" calculated above with the rock masses grades classified by the traditional rock masses rating methods

a new method of rock masses quality classification

which takes "representative fractal dimension" of the discontinuity distribution as the classifying index

is proposed. The equivalent shear strength indexes of rock masses can be calculated

basing on the rock masses quality classification.