Abstract: The engineering classification of rock masses is one of the basic problems in rock mechanics .The main purpose of engineering classification of rock masses is comprehensively to evaluate rock masses ,to divide the rock masses into several proper classes and to offer guantitative data for engineering design .therefore engineering classification of rock masses is very important for engineering practice and aroused a great interest to the investigators of rock mechanics .On the basis of Bieniawski’s geomechanics classification of rock masses , a new method of engineering classification of rock masses has been proposed by means of the theory of modern fuzzy mathematics .In this classification the rock masses are evaluated on the basis of six parameters :the uniaxial compressive strength of the rock material drill core quality RQD, spacing ,orientation and condition of joints ,and ground water inflow. The rating for each parameter is determined and total rating for rock mass is specified by the method of weighted average .The rating and the weight for each parameter were considered as fuzzy variables . Therefore not only the total rating but also varying Range of total rating(or pthe membership of total rating ) are given .The randomly varying state of in situ rock masses can be described by this method of classification so that the result of comprehensively evaluating is more accurate than others . In additional, an accurate and simple evaluating method that is suitable for the engineers has been given .