High quality Mn:ZnSe/ZnSe nanocrystals were successfully synthesized using an environmentally friendly method. Selenium powder was dissolved directly into octadecene to form Se precursor without the assistance of phosphine complexes. To improve the quantum yield (QY) and stability of as-synthesized nanocrystals, ZnS shell was introduced by epitaxy growth technique to form Mn:ZnSe/ZnS core-shell nanocrystals. Absorption spectroscopy, photoluminescence (PL) spectroscopy, X-ray diffaraction, and transmission electron microscopy were used to characterize the property and structure of as-synthesized nanocrystals. The results demonstrated that both Mn:ZnSe and Mn:ZnSe/ZnS have a good monodispersity with cubic zinc blende structures. The PL QY of as-synthesized Mn:ZnSe/ZnS nanocrystals could reach as high as 60%. The fundamental mechanism and effects of ZnS shell thickness and Mn<sup>2+</sup> dopant concentration on the PL emission were also discussed preliminarily.