The deformation and failure of a rock mass subjected to engineering load are essentially controlled by its internal structure, so that the study of rock mass structures should be considered as a foundation on which to understand its behaviour. Accordingly, in this paper, rock mass structures are classified into 4 types: intact blocky structures, layered structures, fractured structure, and loosened structure, and a rock mass quality, Z, is being proposed to evaluate the engineering geological behaviour of rock mass: Z = I. f. S. where I is intactness of rock mass; f is the frictional coefficient of predominant structure surfaces; S is the stiffness of the rock block.