Abstarct: In Underground Coal Gasification (UCG) process, coal seams under different range of thickness, dip and depth without using the conventional coal mining with complicated thermodynamic and chemical processes in-situ by injecting oxidant (air or steam/oxygen mixture) converts into the mixture of combustible gases (CO, H2 and CH4) and incombustible gases (N2, H2O and N2). The resulting gas, called Syngas, is captured at the production well, then cleaned up and be used in many applications, such as the primary use in electricity generation and production of chemicals. Mazino coal mine has about 1400 million tons of thermal coal that don't used in country's steel industry for its coking ability. In this thesis, the prefeasibility study of using the UCG process in Mazino coal mine examined. M26 coal seam of Mazino mine selected as a feasible seam by using the UCG site selection criteria and characterization. One the most important challenge in UCG process is gas seepage of UCG reactor, because the problem in gasification operation and decrease the process recovery, also in this thesis the gas seepage modeled by the numerical method. Therefore, UCG controllable parameters such as gas temperature and operating pressure and uncontrollable parameters including reactor depth and bedrock and bedrock permeability influence investigated. The results show that the gas temperature and reactor depth has a negligible effect, and operating pressure and rock permeability has excess effect in gas seepage.