A new and convenient route is developed to synthesize CdSe and core-shell CdSe/CdS quantum dots (QDs) in aqueous solution. CdSe QDs are prepared by introducing H2Se gas into the aqueous medium containing Cd2+ ions. The synthesized CdSe QDs are further capped with CdS to form core-shell CdSe/CdS QDs by reacting with H2S gas. The gaseous precursors, H2Se and H2S, are generated on-line by reducing SeO3 (2-) with NaBH4 and the reaction between Na2S and H2SO4, and introduced sequentially into the solution to form CdSe and CdSe/CdS QDs, respectively. The synthesized water-soluble CdSe and CdSe/CdS QDs possess high quantum yield (3 and 20 %) and narrow full-width-at-half-maximum (43 and 38 nm). The synthesis process is easily reproducible with simple apparatus and low-toxic chemicals. The relatively standard deviation of maxima fluorescence intensity is only 2.1 % (n = 7) for CdSe and 3.6 % (n = 7) for CdSe/CdS QDs. This developed route is simple, environmentally friendly and can be readily extended to the large-scale aqueous synthesis of QDs.