Diamondlikea‐C:H films have been deposited by decomposition of C2H2. By the combination of Brillouin scattering and the ultralow load indentation technique, the following mechanical properties of the films were studied: the microhardnessH, the Young’s modulusE, the shear modulus &mgr;, and the Poisson’s ratio &ngr;. From the measured data of mass density, hydrogen concentration, and the ratio ofsp3/sp2bonds the filling factors of these films were calculated. The results show that the hardness, Young’s modulus, and shear modulus of these films depend strongly on the volume concentration of voids and are directly proportional to the filling factor.