Adsorption of H2 on functionalized graphite slit-pore at 77 and 298 K was investigated by using a grand canonical Monte Carlo simulation. The physical pore width in a range of micropores was studied. We plotted the adsorbed amount and heat contributions as a function of pore width and pressure. We found that the pore width, pore volume, and functional group play a key role in H2 uptake in different conditions. Pore widths of 0.65 and 1.0 nm are the optimum widths and play an important role at low pressures while the total pore volume is the key at higher pressures in H2 gravimetric capacity at a wide temperature range of 77–298 K. However, functional group properties do not play a significant role in H2 adsorption capacity at 298 K. They play a significant role and become more pronounced at a lower temperature (< 298 K) during low pressures until the functional group becomes saturated by adsorbate. The results obtained with different force fields confirm that N-doped porous carbons are more effective than O-doped porous carbons for enhanced H2 storage. The heat contribution between H2 and functional group type is divided into three levels base on the values which is in the order of quaternary-N/pyridinic-N-oxide > pyrrolic-N/carboxyl > hydroxyl/carbonyl. Moreover, the adsorbed amount and all the heats of the higher concentration of functional group are higher than that of lower concentration of functional group. The saturation of the higher concentration of functional group occurs at a higher pressure, thereby indicating the wider pressure range of enhanced H2 storage. This work provided a new strategy to develop the optimum pore width and suitable functional group type for enhancing H2 storage in porous carbons. [ABSTRACT FROM AUTHOR]