Wettability (by water) of a stainless-steel plate was modified by depositing an ultrathin layer of a plasma polymer of trimethylsilane (TMS) and an oxygen mixture. Plasma polymerization was carried out by cathodic glow discharge polymerization, in which a substrate stainless-steel plate was used as the cathode of the de glow discharge. The plasma polymer of TMS (oxygen mol fraction = 0) is very hydrophobic, but the addition of oxygen in the monomer mixture increased the hydrophilicity of the plasma polymer. The value of cos theta (theta : advancing contact angle of water) linearly increased with the mol fraction of oxygen in the monomer mixture. Oxygen in the monomer mixture acts as (1) an ablating agent of a carbon-based plasma polymer and (2) a comonomer for a silicon-based plasma polymer. The atomic composition of a plasma polymer, which can be expressed as SiCxOy, changes (x decreases and y increases) as the mol fraction of oxygen increases.