The luminescence gap in the regioregular poly(3-hexylthiophene) film is shown to be 1.93eV at 16K from the excitation energy dependence of the luminescence spectrum. The self-trapped exciton, of which the luminescence is due to the recombination, is suggested to consist of a self-trapped hole and an electron bound around the hole. The power law decay described by t(-1.14) is observed, which is explained from the diffusion in the fractal structure of the electron bound around the self-trapped hole.