An attempt was made to simulate carbon gasification with the aid of molecular orbital theory. A simple Huckel method was applied to calculate molecular orbitals of a large polynuclear aromatic molecule, which was employed as a model crystallite for carbon. A free valence at each carbon atom in model crystallites of varying size and shape was determined and this value was taken as a measure of reaction probability for the Monte Carlo simulation. Simulation of carbon gasification was performed by removing the carbon atoms from the model crystallites according to their corresponding reaction probabilities. This simulation made it possible to visualize how the shape of carbon crystallite is changed during gasification. It was found that the specific gasification rate depends on molecular size, but is essentially independent of the geometry of the starting model crystallite.