Bulk silicon (Si) and germanium (Ge) have an indirect band gap transitions however when they arernminiaturized to nanometer scale, the energy gap between the highest occupied molecular orbitalrn(HOMO) and the lowest unoccupied molecular orbital (LUMO) increases, and hence the transitionrnchanges to direct due to confinement. The HOMO-LUMO gap determines the excitation of electronsrnso that the nanostructures will emit light. In this work,quantum confinement effects for Si andrnGe,some methods to calculate band structures and formation mechanisms of photoluminescencernfrom Si and Ge nanostructures are presented. We presented the parameters that influence thernphotoluminescence intensity of Si and Ge nanostructures. Finally we developed a model that couldrnexplain experimental results of Si nanocrystal photoluminescence versus size and wavelength byrnusing Matlab program