Changes in chemical states of amorphous carbon film during ethylene (C₂H₄) plasma in the floating potential were investigated with multiple-internal-reflection infrared absorption spectroscopy (MIR-IRAS) and deposition rates. IRAS spectra showed the peaks due to the sp³-CH_X were observed, but no peaks due to sp²-CH_X were observed. The deposition rate due to ethylene plasma were nearly twice as much as that due to methane plasma, in the same way that the number of carbons in an ethylene molecule is twice as that in a methane molecule. It is suggested that the film growth due to ethylene plasma is the same manner of that due to methane plasma; the plasma-generated hydrocarbon species such as C₂H₃, C₂H₅, which are generated in ethylene plasma are adsorbed on dangling bonds which are generated by hydrogen abstraction from the deposited film surface. As a result, the deposited film is comprised of sp³-hydrocarbon components.