Hydrogenated amorphous carbon (a-C:H) films were deposited on p-type Si(100) at room temperature by DC saddle-field plasma enhanced CVD using pure methane gas and diluted methane gas by inert gas. The effect of substrate bias (Vs) from 0 V to 400 V on deposition rate, hydrogen content, chemical bonding of the films have been investigated. FT-IR spectrum shows that a-C:H films consist of sp2 and sp3 bondings and the relative intensity ratio of sp2 and sp3,I(sp2)/I(sp3), decreases from 0.28 to 0.19 as the substrate bias changes from 0 V to 400 V. Deposition rates of the films rapidly decrease with increasing Vs up to 200 V, and then slightly decrease over Vs = 200 V. The hydrogen content of the films increase as Vs goes higher and the number of C-H bonds of a-C:H shows the same trend. Only in the range of Vs=0 V and Vs=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 PL has a maximum at Vs=200 V, then decreases as the Vs changes from 200 V to 400 V. The PL peak shifts to higher energy from 2.15 eV at 100V to 2.45 eV at Vs=400 V. The PL at Vs = 400 V was two times stronger without a peak shift by using an inert gas dilution into methane source compared with that of a film deposited using pure methane.