Abstract: Three parameter yield criterion to simulate the lower tensile property of rock mass is presented in this paper. It brings out three parameter elasto plastic computational model with MC(or DP) yield criterion. The disadvantage of the conventional MC(or DP) criterion with tensile cut off unable to deal with the lower tensile property of rock mass is overcome by this model. The envelopes of softening curves for post yielding of rock mass materials are also presented to give the complete computational model. The "Back Euler" stress integration algorithm and the consistent elasto plastic matrix for this model are also given for assuring the Newton Raphson interation convergence to be of the second order.