We have studied time response of self-trapped exciton (STE) luminescence in anatase phase of TiO2 as functions of excitation photon energy and temperature. The decay curves, which consist of two components obeying exponential and power-law decay, are found to depend significantly on the two parameters. The exponential component is stimulated efficiently under excitation near the absorption edge. This fact suggests that under such an excitation condition, photogenerated electron-hole pairs directly form STE's. It is found from temperature dependence of the decay curves that the exponential component is thermally unstable compared with the power-law one. It is inferred that the electron-hole pairs are thermally separated, and thus the direct formation of STE's is prevented.