We have studied the stability of amorphous silicon and polycrystalline silicon films under illumination. These films are prepared by plasma-enhanced chemical vapor deposition technology from SiH4/H2 and SiCl4/H2 separately. The exp eriment indicates that the light-soaking degradation phenomenon, which exists in almost all amorphous silicon films, does not appear in the polycrystalline sili con films. The light-dark conductivity of polycrystalline silicon films does not decrease but increase during light irradiation. Furthermore, the variance of co nductivity depends on hydrogen dilution ratio. It is suggested that the persiste nt photoconductivity effect of polycrystalline silicon films may originate from the high crystallinity and the action of chlorine.