Coated CdSe/CdS nanocrystals have been synthesized through a new reaction routine in micelle solution. High resolution transmission electron microscope image demonstrated the monodispersity and core-shell structure of CdSe/CdS nanocrystals. Photooxidation experiments shown that by coating with a wider bandgap material CdS, the surface states of CdSe cores could be decreased and the photo-instability of their luminescence spectrum could be reduced. This phenomena were temporary interpreted that lack of carriers at the outlayer surface reduced photooxidation degradation of CdSe/CdS nanocrystals.