La Ni O thin films of different La and Ni concentration ratios were deposited on (111)Si at substrate temperature 260℃ by a rf magnetron sputtering system with a combined target. The optical refractive index and extinction coefficient were derived by simultaneously fitting the transmittance and reflectance spectra in the wavelength region of 2～ 12.5 μm.And they increase monotonically with the wavelength, and change significantly with different composition. Results also show that the film is of amorphous structure when La content is too high. The film is with (100) preferred orientation and metallic electric conduction as the atomic concentration ratio of La to Ni is less than 1.44∶1. When the composition gets the stoichiometric composition(Ni∶La=1∶1), the lattice constant and sheet resistivity of the film reach their minimums. A practicable explanation for all experimental results is also developed according to the conducting mechanism of LaNiO₃.