The atmosphere of Mercury is of an exospheric nature. Its formation is due to several physical mechanisms including meteoroid impact, surface sputtering by solar wind ions and photon sputtering by solar UV radiation. The molecules and atoms emitted from the surface materials of Mercury include H, He, O, Ar, and S, etc. It is important to study their spatial distributions across the planetary surface via ballistic random walk. We have developed a surface thermal model coupled with Hodges-type Monte Carlo calculations to simulate the exosphere of Mercury, which will be a major scientific target of the BepiColombo mission of ESA and JAXA.