Abstract: Nanocrystalline silicon thin films were deposited by radio frequency（RF） sputtering on p-type silicon substrates at low temperature（95℃）.In the working gas（H²+Ar）,the H₂ dilution percentage was varied from 31% to 70%.The duration time of film deposition was changed at the same time.The optical band gaps（E_g^opt） were derived from Tauc plots,and the result showed that the films prepared by RF-sputtering has a wider band-gap.Basing on our experimental results and the heterojunction quantum dot（HQD） theory,a new modified band gap model of the nc-Si:H film was suggested.The I-V characteristics about the nc-Si:H film were discussed theoretically and experimentally.