The outcome of particle size effect on Photoluminescence has been investigated theoretically for CdS, ZnS, CdSe and ZnSe quantum dots. The theory is based on computational modeling in the strong confinement region only where the particle size is less than Bohr's radius. The Photoluminescence emission line shape function which depends on band gap was found to be strongly dependent on particle size. The PL spectra of bulk material is similar for the quantum dots where similar vibronic coupling and normalized intensity is considered, only the peak wavelength changes and thus shifts the band edge luminescence peak to higher energies for the quantum dots corresponding to their band gap with no broadening. The PL spectra of monodisperse dots revels the fact that the shape of the emission peak is same, only it shift towards higher energy or smaller wavelength for decreasing size of quantum dots.