The size dependence of photoluminescence (PL) from nanostructure semiconductors is examined. Considering the dependence of PL on both the silicon nanoparticles (Si NPs) sizes and their dispersion, we incorporated quantum confinement effects along with the effects of localized surface states to obtain an analytical expression for the PL spectra of silicon nanostructures. In order to obtain an insight into the effects of various parameters influencing the PL spectral profile in silicon nanostructures, we computed the PL spectra using relevant numbers in the expression. The computer-simulated results show (i) a marked deviation of PL spectrum from the normal distribution at higher energies due to the increase in oscillator strength with the decreasing mean Si NP size, (ii) The peak position redshifts and the peak intensity reduces with an increase in the standard deviation, and (iii) the luminescence peak blueshifts as the mean Si NP size decreases. To test our model, the Si NPs embedded in silicon nitride films were prepared by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique using the H<sub>2</sub> diluted SiH<sub>4</sub> and N<sub>2</sub> as reactant gas sources. The simulated PL spectra fit the experimental one rather nicely. And our results can explain the reported experimental observations on the luminescence from Si NPs.