First principles calculations are carried out for investigating effect of dot-size and hydrogen passivation in silicon (Si) quantum dots. In this work, the electronic and optical properties of pure and hydrogen passivated Si quantum dots are explored. However, the importance of quantum confinements and hydrogen terminated surface on the energy gaps and optical absorption are discussed. Our results exhibit that the hydrogenated surface can cause modification in the electronic structure of Si quantum dot. It is found that the band gap increases as a function of size reduction in both passivated and unsaturated Si quantum dots because of quantum size effects. For the passivation of surface dangling bonds with hydrogen atoms, the energy gap is larger than that of pure Si quantum dots. Passivation with hydrogen atoms has a significant effect on the spatial distribution of the highest-occupied and lowest-unoccupied molecular orbitals. The impact of hydrogenation and dot-size on the optical absorption spectra and static dielectric constant is also inspected. Precisely, the dependence of dot-size and hydrogen passivation on the absorption threshold is elucidated. We surmise that this theoretical contribution can be valuable in discerning the microscopic processes for the future realization of nano-optoelectronic devices. (C) 2013 Elsevier B.V. All rights reserved.