Epitaxial and polycrystalline silicon layers on sapphire have been annealed with Q-switch pulses from a Nd : YAG laser irradiated on the Si surface. Time-resolved optical reflectivity measurements have been performed. The annealing process is shown to be induced by melting and subsequent epitaxial regrowth. The best results were obtained if the whole Si layer was melted, thus allowing the (11̄02) oriented sapphire substrate to act as a seed for recrystallization. In this case commercially available Si-on-sapphire (SOS) wafers with additional Si implantation as well as polycrystalline layers of low-pressure chemical vapor deposited (LPCVD) Si on sapphire could be epitaxially regrown. The same material deposited on amorphous SiO2 did not show epitaxial regrowth, however an increase in size of randomly orientated grains from 60 nm to 1 μm could be achieved.