The photoluminescence quantum efficiency of semiconductor nanocrystals at room temperature was markedly enhanced about 50 times by surface treatment of the nanocrystals after synthesis, and the original photoluminescence peak position and narrow line width were maintained. Simply, the nanocrystal surface was treated with a small amount of sodium borohydride (NaBH4) in toluene at room temperature. The surface-treated nanocrystals showed very high quantum efficiency, up to 75%, even in the blue region of 490 run. The TGA and XPS data revealed that some of the surfactants on the nanocrystal surface were removed by the reaction with NaBH4 and that the exposed cadmium on the surface was converted to cadmium oxide. The cadmium oxide functioned as a robust passivation layer on the CdS surface and contributed more electron density toward the inside of the CdS core to enhance exciton recombination. The surface treatment was powerfully effective in improving the quantum efficiency Of II-VI compond semiconductors such as CdS, CdSe, and CdTe, and generally their mixture-type nanosized materials synthesized under a variety of conditions.