Micro-structural changes and decomposition of hydrogenated carbon films on magnetic disk substrates prepared by DC-magnetron sputtering were studied using Ar+ laser irradiation and physical heating in air and nitrogen ambience, respectively. It was found that a weak Ar+ laser of a fluence of 0.3 mW μm-2 could significantly decompose a-C:H films. The decomposition rate was fast initially and slowed down significantly after about 20 min. The decomposition rate of a-C:H increased with hydrogen concentration in the film. Raman light intensities of the irradiated sites reduced exponentially with irradiation time. Physical heating in air revealed that the oxidation-dominated decomposition took place between 320 and 350 °C within 15 min, and an increased hydrogen concentration made the a-C:H film more vulnerable to decomposition. At 150-300 °C, a phenomenon of nano-crystal graphite formation was observed from Raman spectra. XPS analysis indicated that there was no significant diffusion of carbon atoms into the substrate at 400 °C heating.