Abstract: Phosphor doped (n-type) hydrogenated amorphous silicon (a-Si:H) thin films were deposited by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). The refractive index, extinction coefficient, optical bandgap and hydrogen content of the testing films were investigated by means of ellipsometry and Fourier transform infrared spectra (FTIR) with gas pressure varied from 20 Pa to 80 Pa. The microstructural changes of a-Si:H thin films caused by different gas pressure were studied and discussed using Raman spectra along with optical property measurement. The results show that the optical bandgap and the hydrogen content of a-Si:H thin films increase with the rise of gas pressure in PECVD system, while the refractive index and the extinction coefficient decrease gradually. In the mean time, the ordering of amorphous network of the testing thin films on the short and intermediate scales is getting worse as the gas pressure goes up.