Abstract: The microcracks in samples of Inada granite after triaxial experiments were examined by microscope. The microcracks in two types of triaxial experiments, covention-al triaxial (1>2 = 3) and true triaxial (1>2>3) were compared. The observation showed that the anisotropic dilatancy effect of brittle rocks as revealed by K. Mogi in true triaxial experiment was caused by aiiisotropic microcracks in rocks, thus indicating its microscopic nature. The effect of anisotropie dilatancy and anisotropic microcracks caused by the principal stress of intermediate magnitude with regard to earthquake studies was preliminarily discussed.