Stearic acid is treated in a DC Ar-O2 plasma created by a cathode-anode confined system. The influence of the most important process parameters (gas flow rate, sample temperature, output power and exposure time) on the acid modification is studied. The evaluation of the influence of these parameters on grafting and etching of stearic acid was done by measuring the mass variation rate (MVR). The results show that when charged and chemically active species increase in density, what is directly connected with plasma parameters, the MVR increases too. In all experimental conditions, a negative MVR was obtained, due to the etching of the sample. The etching rate decreases with processing time, probably because of the formation of a product which is more resistant to plasma etching.