The epoch of reionization (EoR) is the first billion years in the cosmic history when the first generation of galaxies were ionizing the intergalactic medium. So far, this epoch was too complex for computers to simulate and too far away for the telescopes to see. With technological advance in recent years, however, we are now becoming able to explore this interesting epoch with both state-of-art simulations and observations. I first introduce how we simulate the EoR and what observables we can probe with large-scale (> 100 megaparsec) simulations. My main interest is to improve the accuracy of such simulations by supplementing unresolved small-scale (< 1Mpc) recombination effect as sub-grid physics. This recombination effect is also a key ingredient for understanding how many galaxies are needed to reionize the universe. I present a small-scale simulation designed to model the recombination rate and describe the results from the simulation. Photo-heating from ionization quickly destroys the small-scale structure to suppress the recombination rate, but the structures achieve significant amount of recombination before they get destroyed. I discuss expected impacts of the small-scale recombination on the global reionization.
