In this study, numerical computations were performed to scrutinize the effect of the size of coal particles on reactive flow fields and concentration distribution of product gases for five cases with four different particle sizes (40 $mu extrm{m}$, 60 $mu extrm{m}$, 100 $mu extrm{m}$, 120 $mu extrm{m}$, 140 $mu extrm{m}$) in an axisymmetric cylindrical coal gasifier in which Alaska Usibelli subbituminous coals were gasified. Predictions showed that coal particle size affected the concentration distribution of product gases. When coals of 100 $mu extrm{m}$ were gasified, the maximum average concentrations of major products, H$_2$and CO, were predicted at the exit of the gasifier. The average mole fractions of CO and H, were shown to be 0.62, and 0.16 (dry basis, inert free), respectively. The cold gas efficiency of 83% was also predicted for the same particle size of 100 $mu extrm{m}$.