A series of hydrogenated amorphous SiC (a-SiC:H) films that contain Si quantum dots (QDs) were grown using PECVD at a substrate temperature of 320 degrees C by changing the deposition time from 0.5 h to 3 h. The chemical composition, phase constitution, microstructure and optical gap of the films were systematically studied by X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, and UV-vis spectrophotometer, respectively. It was shown that long time deposition was equivalent to introduce an annealing process, which enhanced the Si QDs growth and the formation of gradient size distribution. With increasing the deposition time from 0.5 h to 3 h, the optical band gap E(opt) decreased from 1.92 eV to 1.45 eV. It was due to the reduced quantum confinement effect when the size of Si QDs increased from -1.5 nm to -4 nm. These results provide the experimental basis for the growth and controlling of Si QDs.