Abstract: The basic theoretical background, control equations and computation principle for solving thermo-hydro-mechanical (THM) coupling problem in UDEC Version 4.0 of 2D distinct element method are presented. The numerical simulations for the coupled hydro-mechanical (HM) and THM processes in near field of a conceptual disposal repository for nuclear waste are carried out by UDEC assuming the repository to be located a rock mass in which two sets of joint and underground water exist. The investigation is made for the distributions of displacements, principal stresses, plastic zones, temperatures, joint apertures, water pressures, water flow rates and temperature changes with time in the surrounding rock mass of the drift. The results of calculation show: compared with the case of HM coupling, the heat relaxation of nuclear waste in THM coupling makes the obvious changes of magnitudes and directions of the stresses and the displacements in the surrounding rock mass with water of the disposal repository, and causes the decreases of joint apertures, of water pressures and water rates in the rock mass, especially of the middle part of the drift wall. If a disposal repository for nuclear waste in jointed rock mass is intended to be designed, constructed and operated safely and reliably, it is very important to analyze the coupled THM processes concerned.