This presentation will discuss the correlation of band structure with charge separation, charge migration and charge recombination in the semiconductor-based photocatalysts. Doping elements into the semiconductor could alter the electronic band structure, which will change the charge separation and charge recombination processes, leading to modulation of photocatalytic activity. When two materials are formed to a heterojunction, the band alignment and the interfacial defects in the heterojunction have significant effects on the charge separation, charge migration and charge recombination processes.