The effects of Ar addition on the structure of a-C:H films have been studied in inductively coupled CH4/Ar plasmas. The deposition rate was determined carefully with an incubation time prior to the film growth. The compressive stress and bonding properties of the films, and the emissive species and total ion flux in the plasmas were examined to find out the relation between the film structure and the bombarding species and Aux. The stress increased for the Ar content in the feeding gas from 0 to 0.6, then drastically decreased, together with an increase in the sp(2) phase, for the Ar content exceeding 0.6. The structure changes in the films accompanied by the stress change is discussed in terms of the two competitive Ar content dependent processes; the densification by Ar incorporation into the films and the damage production especially caused by Ar+ bombardment.