ZnO powders with different morphologies were synthesized by alkali precipitation method, organometallic zinc hydrolysis, and spray pyrolysis. Acetaldehyde decomposition was used as a probe reaction to evaluate the photocatalysis of these ZnO powders. The relationship between photocatalytic activity and crystallinity, surface area, or morphology was investigated. The results indicate that the photocatalytic activity depends on crystallinity, rather than surface area, for the same kind of ZnO particles. However, no direct relationship between photocatalytic activity and crystallinity or surface area was found for the different kinds of ZnO particles. Instead, we find that the particle morphology significantly affects its photocatalysis.