Abstract: The conditions, process and mechanisem for the step formation are given in the paper. According to the nolinear relationship between displacement of a elastic body and its size derived under the action of its body forces from the similarity theory, an expression between size of a fault step and its active length is derived for the case of plane strain. Attempts to confirm this quantitative relationship by modelling specific examples numerically and physically is made by the authors and analysis of the results of these studies is given in the paper.