We employed the constant photo-conductivity method (CPM) to measure the optical absorption spectra of nano-crystalline siilicon films (nc-Si :H) in the energy range of 0. 9-2. 5eV. We analysed some possible major optical transition processes resulting from the photo-conductivity at different photon energy range and the variation of optical absorption spectra while the films were transformed from amorphous silicon into microcrystalline silicon and then nano- crystalline silicon films. We found that the carriers' optical transition and transport process at the interfaces between grains play an important role in the optical absorption spectra in the whole energy range. We make use of the novel structure of nc-Si:H films to explain our experimental results.