Film deposition process during helium-added acetylene plasma is investigated with multiple-internal-reflection infrared absorption spectroscopy (MIR-IRAS). The sp² -C, the sp-C, the sp³ -CH, -CH2, and the sp-CH components are formed in the deposited film. We found that the films are deposited through the addition reaction during the plasma. We also suggest that the amounts of the sp³ -CH, -CH2 components are etched during He-added acetylene plasma.