CO2 adsorption on carbon is a CO2 geo-storage mechanism, and a potential technique for CO2 removal from flue gas or pressurized fuel gas streams produced from hydrocarbon reservoirs (which frequently contain CO2, sometimes at very high concentrations). However, the detailed mechanism of precisely how CO2 is adsorbed on the carbon surface is only poorly understood. We thus simulated supercritical CO2 adsorption in carbon slit pores at the molecular level by Grand Canonical Monte Carlo calculations. Adsorption isotherms and microscopic structural properties were examined for different pore widths, pressures, and temperatures. Our results demonstrate that the excess adsorption density isotherm of supercritical CO2 in a carbon slit pore has a maximum value, and it is not a monotonically increasing function of pressure. However, supercritical CO2 cannot be effectively adsorbed at very high temperatures (>= 850 K) as the excess adsorption density is extremely small. Mechanistically, multiadsorption layers were observed in large slit pores (pore width = 20 angstrom); these were defined as contact layers, inner layers, and free layers, respectively. Finally, the optimum pore widths for supercritical CO2 adsorption under different conditions were determined, which is vital for optimizing adsorbent and CO2 geo-storage efficiency.