The stability of the process of underground coal gasification and its gas compositions depend on, to a large extent, the features of the convection diffusion of the gas and the dynamical conditions of chemical reactions. The dynamic distribution of the gasification agent concentration, in particular, has a great influence on the combustion and gasification reactions. In this paper, the basic features of convection diffusion for the gas produced in underground coal gasification are studied. On the basis of the model experiment, through the analysis of the distribution and patterns of variation for the fluid concentration field in the process of the combustion and gasification of the coal seams within the gasifier, the 3-D non-linear unstable mathematical models on the convection diffusion for oxygen are established. Additionally, the determination method of the major model parameters is explained. In order to curb such pseudo-physical effects as numerical oscillation and surfeit frequently occurred in the solution of the complex mathematical models, the novel finite unit algorithm-the upstream weighted multi-cell balance method is adopted in this paper to solve the numerical models established. The author also analyzed and discussed the simulated calculation results, which show that, except very few points in loosening zone, where the relative calculation error is comparatively high (>20%)resulting from the low oxygen concentration, the relative calculation error of other points falls between 7% and 17%. Therefore, the calculation value and the experiment value take on a good conformity. According to the simulated results, the calculation value of the oxygen concentration is a little bit lower than the experiment one. On top of that, with the prolonging of gasification time, in high temperature zone, the change gradient of oxygen concentration for experiment value is bigger than that of the calculation value. The oxygen concentration is in direct proportion to its distance from the flame working face, i.e. the longer its distance is, the higher the oxygen concentration is; otherwise, the lower. In the vicinity of the combustion zone, due to the very high temperature, the oxygen is almost exhausted in the reaction with carbon; in loosening zone, the oxygen concentration drops to a very low point where it almost approaches zero; in dropped out zone, owing to the comparatively low temperature, the drop of the oxygen concentration is slow. Research indicates that the simulation value can conform with the experiment value relatively well, which demonstrates that the numerical simulation on the oxygen concentration field in the process of underground coal gasification is reasonable. It also provides important scientific theoretical basis for the farther quantitative study and the prediction of the patterns of variation in the process of underground coal gasification. (C) 2004 Elsevier B.V. All rights reserved.