Mechanical and optical properties usually play crucial roles for the protective coating of optical lens or plastic case. High hardness, low surface friction, and high optical transmittance are all superior characteristics of diamond-like carbon (DLC), which, therefore, become an excellent choice as a protective coating. This study prepares thin DLC films silicon and polycarbonate (PC) substrate by an unbalanced magnetron (UBM) sputtering system. The thin DLC films deposited on the PC substrate are successfully prepared at low working temperature, 80 degrees C, and show a good adhesive characteristic. Low working pressures, less than 1 mTorr, are proven to be helpful in producing DLC films with improved mechanical prosperities and microstructure qualities. The obtained films have an average hardness of 12 GPa and average roughness of less than 0.3 nm. The measured transmittance is 85% within visible light range, at the expense of mechanical properties, for DLC films prepared under a working pressure of 50 mTorr and a radio frequency (R.F.) power of 200 W (c) 2005 Elsevier B.V All rights reserved.