The dependency of the photoactivity on the crystallite size of anatase titania for the decomposition of trichloroethylene (TCE) was investigated. The crystallite size of anatase-phase was controlled at a fixed surface area and particle size by the spray pyrolysis. The surface area was also changed by the gas-phase decomposition of titanium-tetra-iso-propoxide (TTIP) by keeping the crystallite size in a similar range. The photoactivity of titania particles of the same surface area and particle size increased linearly with increasing the crystallite size of anatase-phase as long as no significant rutile phase was formed. It was proved that the photoactivity of titania is supposed to be enhanced by increasing the surface area while keeping a similar crystallite size. The lab-made titania particles of nanometer size and pure anatase-phase had higher photoactivity than that of Degussa P25 due to the larger crystallinity, although their surface area was smaller than that of Degussa P25. For all titania samples, the photoactivity normalized by the surface area was linearly increased with increasing the crystallite size of anatase-phase, regardless of the preparation method; the particle size ranging from nanometer to micrometer.