Abstract：Making full use of the results of two-dimensional research and the excellent properties of Genetic Algorithm and Monte-Carlo Method，a method is presented to find the combinations of joint and rock bridge associated with the least comprehensive shear force in simulated 3D joint maps and determine the shear strength and persistence property of jointed rock masses. This method is able to reflect the main factor which controls mechanical properties of a rock mass，for it takes into account the distribution characteristics of 3D joints. The calculation results of two examples are in accordance with the results of back analysis，and the method is proved to be reasonable.
