We present the Raman spectra of amorphous hydrogenated carbon nanoparticles prepared in an Ar–CH4 hydrocarbon plasma. The measured Raman spectra are compared with that of both soft polymerlike and hard diamondlike a-C:H films deposited from pure CH4 plasma. The film properties were studied by Rutherford backscattering and spectroscopic ellipsometry. It is shown that the plasma characteristics are continuously changed from a “precursor-rich” state to a “precursor” state during the particle formation process. As a result, the net flux of the carbon atoms to the surface of the nanoparticles (growth rate) decreases linearly as a function of time as the nanoparticles are growing up. Also, the hydrogen content in nanoparticles increases as a function of nanoparticle size. The particle growth process is then examined by a simple ballistic model. It is found that the volume polymerization process, i.e., the growth of the nanoparticles at a large particle size (⩾100nm), is essentially the same as the surface polymerization process occurring at a surface of a substrate.