Semiconductor nanocrystals appear different optical properties from their bulk analogous due to quantum confinement effects. It is well known that optical absorption of nanocrystals shift to higher energy as the sizes decrease, while the emission at the absorption band edge is always too weak to be investigated in details due to much nonradiative recombination at nanoparticle surface. To eliminate the surface defects, some kinds of materials such as organic molecules, polymers and inorganic compounds are used to overcoat the nanoparticles. It has been found that nanocrystals overcoated with high band gap inorganic materials maned core/shell type nanocrystals exhibit high photoluminescence intensity at band edge due to effective elimination of surface nonradiative recombination. In this article we report the characteristics of CdSe/CdS core/shell nanocrystals synthesized with mercapto acid as stabilizer. The effect of CdS shell on the electronic characteristics and the luminescence of CdSe core were investigated by using optical absorption, photoluminescence and photoluminescence excitation spectra. Large band edge luminescence enhancement was found in CdSe nanoparticles overcoated with the CdS shell with a thickness of 0.3 nm due to passivation of nonradiative recombination sites on CdSe surfaces. Such bandadge emission also appeared redshift from the absorption maximum. This Stokes shift of the emission peak from the absorption edge was explained by a model of formation of excimer among the small nanoparticles based on the optical experimental results and the theoretical calculation on fine structures of bandadge excitons.