Nanocrystalline silicon(nc-Si) clusters can be formed by rapid thermal annealing the hydrogenated amorphous silicon (a-Si:H)films at 600℃-620℃ for about 10 s. Characterized by micro-Raman scattering, the formed nc-Si clusters are randomly distributed in the amorphous matrix of the annealed films with their sizes in the range of 1.6-15 nm in diameter. Under high power laser irradiation, in situ nanocrystallization and subsequent nc-Si growth in the a-Si:Hfilms have been demonstrated. Based on the theory of crystal nucleation and growth, the effect temperature ramp rate on the sizes of the formed silicon particles is discussed. Under high power laser irradiation, in situ nanocrystallization and subsequent ncSi clusters are small enough for visible light emission, authors have not detected any visible photolum inescence (PL) from these nc-Si clusters before surface passivation. After electrochemical oxidization in hydrofluoric acid, however, intense red PL has been detected. Cyclic hydrofluoric oxidization and air exposure can cause subsequent blue shift in the red emission．The importance of surface passivation and quantum confinement in the visible emissions has been discussed.