In ion-assisted plasma enhanced chemical vapor depositions (PECVD), the applied RF power affects both plasma phase reactions and deposited film structure. These dual effects are investigated in PECVD of a-C:H films by using styrene and Ar mixture. The films are deposited in a 13.56 MHz, RF driven asymmetric plasma reactor at 25 degrees C. In situ impedance analysis is used to estimate the ion energy and flux at the substrate, while an on-line mass spectrometer (MS) is used to analyze the plasma chemistry, and film structure is characterized by Fourier transform infrared (FTIR) spectroscopy and ellipsometry. The impedance analysis shows that both ion energy and ion flux increase with increasing RF power. The MS data show that the RF power has the major effect on the degree of dissociation of styrene. The FTIR spectra of the deposited films indicate that CH, is likely to be the precursor for the diamond-like carbon deposition. The ion energy flux (IEF) is shown to have a significant effect on the film structure. With increasing IEF, films show decreases in hydrogen concentration, increases in sp carbon fraction, and increases in refractive indexes.