To elucidate the exciton dynamics at low exciton density, we have carried out measurements of the absorption, luminescence, luminescence excitation spectra, and time response of the luminescence intensity of poly(di-n-hexylsilane) (PDHS) film at 2 K. A broad absorption peak is observed at 3.40 eV and a sharp luminescence line at 3.37 eV. In the luminescence excitation spectra, the luminescence peak energy shift is found with the excitation energy below 3.385± 0.005 eV. This fact shows that the site selective excitation is observed below the excitation energy, and indicates that the energy relaxation rate of excitons changes dramatically around the energy. To understand the above results, we have formulated the exciton dynamics using a one dimensional Frenkel exciton Hamiltonian with disorder. We have also carried out measurements of the time response of luminescence intensity using the up-conversion technique in order to elucidate the exciton dynamics at high density. We have clearly observed the rise time of 1.4±0.5 ps, which is fifty times as fast as that at low exciton density. The decay time is also faster. Moreover, the luminescence quantum efficiency at the exciton density is ten times smaller than that at the low density. A process to explain these results is considered.