Amorphous hydrogenated carbon (a-C:H) films have been deposited in Microwave Multipolar plasma reactor (MMP) using pure acetylene as precursor with controlling substrate temperature. Investigations on the optical properties of deposited films according to the substrate temperature have been reported. It has been observed that the optical band gap decreases and the Csp2 concentration increases when cooling down the substrate temperature. On the other hand, it has been revealed elsewhere that cooling down the substrate temperature during the deposition process, leads to the increase of particles trapped density in the deposited films due to a phenomenon of thermophoresis. Therefore, the optical properties evolution is linked to the increases of particles trapped density in the deposited films.