Hydrogenated amorphous-carbon (a-C:H) films were deposited on p-type. Si(100) at room temperature by de saddle-field plasma-enhanced chemical-vapor deposition using pure methane gas. The effect of substrate bias (V-s) from 0 to 400 V on deposition rate, hydrogen content, and chemical bonding of the films have been investigated. Fourier transform infrared spectrum shows that a-C:H films consist of sp(2) and sp(3) bondings and the relative intensity ratio of sp(2) and sp(3) [I(sp(2))/I(sp(3))] decreases from 0.28 to 0.19 as the substrate bias changes from 0 to 400 V. Deposition rates of the films rapidly decrease with increasing V-s up to 200 V, and then slightly decrease over V-s = 200 V. The hydrogen content of the films increases as V-s goes higher and the number of C-H bondings of a-C:H shows same trends. Only in the range of V-s = 0 V and V-s = 100 V, the Raman band of graphite and disorder is observed. The emission of white photoluminescence.(PL) light from the films is observed with the naked eye even at room temperature. The intensity of the PL has a maximum at V-s = 200 V, then decreases as the V-s changes from 200 to 400 V. The optical band gap (E-g) determined by the Tauc relation is shifted from 1.48 to 2.9 eV with increasing V-s, while the peak position of the PL spectra is shifted slightly to higher energy.