Abstract：With introduction of random probability into the block theory and considering a random distribution of geometry parameters and mechanics intensity of block，this paper used Monte Carlo method to conduct random simulation and calculate failure probability of block of different sizes under different failure modes. In the course not only the mechanics intensity，i.e. friction coefficient and cohesion，were taken into account，but the geometrical parameters such as angle of inclination of structural plane，volume of block，area of slip plane and so on were considered. Thereon the formation probability of different block sizes was calculated in the case that structural plane is discoidal. This paper put forward the concept of association probability which could be achieved from failure probability and formation probability of block，then the relationship between association probability and block size could be deduced. After subarea of block according to size，the association probability of distinct block regions was acquired and the block stability within distinct block regions was analyzed synthetically，which offered theory foundation to the reinforcement of jointed rock slope with significant application value.
