Plasma polymerization of acetylene was performed by using a pulsed rf discharge. Capacitively coupled discharge was initiated in a box type reactor at a uniform flow of an acetylene/argon mixture gas, A 2-10 nm scale ultrathin hydrocarbon film was deposited on a silicon wafer. The distribution of thickness was measured by an ellipsometer. The thickness increased nonlinearly with distance from the monomer inlet. This means that secondary gas phase products took part in the film formation.