For studying plasma polymerization of styrene, two in situ diagnostic methods, optical spectroscopy and mass spectroscopy, were used to measure chemical components formed in the discharge volume and their concentrations in plasma column and two sheaths. The synergetic influence of power (W), pressure (p), and monomer flow rate (F) on plasma polymerization was expressed with a composite parameter, W/pF, which is proportional to the energy transferred to styrene monomer molecule. In a certain range of W/pF, the population of C2H2 and H-2 produced in the discharge decreased with W/pF, while the concentration of C-8 and C-6 fragments increased, which indicates that different chemical reactions may occur in different intervals of W/pF value. The similarity in change tendency between the deposition rate, the emission intensity of CH and C4H2+ and mass peals. vs. W/pF implies that the polymerization is controlled by the reaction in the gas phase plasma, and supports the view that initial reactive species are produced in plasma, and polymerization is performed on the substrate surface.