A new method is proposed to calculate the Stark shift induced by surface dielectric effect in colloidal nanocrystals. The effective mass approximation model is revised according to quantum confined dielectric effect. LUMO (the lowest unoccupied molecular orbital), HOMO (the highest occupied molecular orbital), band gap and Stark shift are calculated in CdSe and PbSe nanocrystals that bear significantly different physical properties. The calculated results fit well with the experimental data. The calculation of dielectric effect-induced Stark shift indicates that the quantum confined dielectric effect in PbSe and CdSe nanocrystals is size- and temperature-dependent, which is more notable in PbSe nanocrystals with a narrower band gap and results in the gentle variation of quantum confinement energy with particle size.