Diamond-like carbon (DLC) films were deposited on radio-frequency (RF) biased substrates at low temperature by electron cyclotron resonance microwave plasma chemical vapor deposition using CH4-Ar as reactant gas. The effects of gas composition ratio, microwave power and RF bias on growth rate, structure and hardness of the films were investigated. Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and Vickers microhardness tests were used to determine the structural change and properties of the DLC films. By changing the microwave power, the growth rate shows a maximum value of 1.6 mum/h at 150 W. From Raman spectra of the films deposited at different microwave power and RF bias, the peaks centered at 1540 +/- 20 cm(-1) for the deposited films, which was the characteristic peak of DLC coatings. By increasing the RF bias the CI-I,, peaks in the FTIR spectra decreased because of hydrogen evolution. The film hardness increased with the increase in microwave power and RF bias. DLC films synthesized at a bias voltage of - (100-250) V and CH4/Ar gas ratio of 2/7 exhibited extreme hardness of more than 3000 kg mm(-2). The role of adding Ar in reactant gases on DLC deposition was also discussed.