Coal oxy-fuel combustion requires CO2 dilution (recycling) to reduce the furnace temperature for material protection. This enhanced CO(2 )concentration reduces the O-2 reaction rate while CO2 gasification contributes to the char reactivity. These conflicting influences together with a competitive behavior between O-2 and CO2 complicate char O-2/CO2 combustion behavior. Individually quantifying the influences accompanying the replacement of N-2 with CO2 and clarifying the competitive behavior are essential for predicting the char O-2/CO2 combustion rate, but this is difficult to obtain via experimental approaches. Here, a simplistic atomistic simulation investigated the char oxy-fuel combustion behavior. In simulations, the reactants (O-2 and CO2) were either permitted to both react with char or permitted to have only one reactive gas with the other being a passive (nonreactive) gas to explore reactivity for individual gases in a O-2/CO2 atmosphere. Thus, gasification contribution to char reactivity was isolated and quantified, and the synergy between O-2 and CO2 reactions was evaluated. Individually quantifying the CO2 influence on O-2 diffusion was also achieved by comparing the simulation of reactive O-2 in passive CO(2)and N-2. The variation trend in char size and apparent density is similar to the experimental observation and other modeling work in zone II. Replacing N-2 with CO2 decreased the char oxidation rate by 8%, being attributed to the reduction of O-2 external diffusion. Competitive behavior between O-2 and CO2 was observed, where the O-2 and CO2 reaction rate was respectively reduced by 10% and 45% from the reactions alone (the other gas being passive). The overall char reactivity for O-2/CO2 is only 78% that the sum of individual gases but is 8% higher than O-2/N-2 under 21 vol % O-2. Simulation under a higher O-2 fraction (50 vol %) and lower gas pressure (20% of the gas molecules) were also conducted. These simulations are consistent with the emerging rationalization of contributing factors to char O-2/CO2 combustion.