A 3D CFD model of the UCG process is developed and evaluated on a small-scale coal block with the dimensions of 3 x 1.5 X 2 cm. The results are discussed in detail and compared with the experimental data available. It was observed that the process reaches a steady sate condition after 4000 s of the process and remains in this condition until 8300 s, when drying and pyrolysis of the whole block is finished. The rate of cavity growth and the temperature at the outlet increased right after this period. Comprehensive sensitivity analysis indicated that the coal thermal conductivity has a major effect on the UCG process. Initial permeability of the seam also had strong influence on the growth rate and shape of the cavity. The higher coal permeability led to faster growth of the cavity and resulted in a much wider cavity. Lower oxygen flow rates at the inlet led to a more bulbous cavity shape, whereas at higher flow rates, the cavity became elongated in the direction of the production well. Increasing the inlet flow beyond a certain value decreased the concentration of CO and H-2 in syngas due to excessive combustion of these gases.