Semiconductor ZnSe quantum dots (QDs) were hydro thermally prepared by the reaction of Zn2+ and NaHSe in the presence of N-acetyl-L-cysteine as a stabilizer. The absorption bandwidth depends strongly on the pH of the working solutions. The photoluminescence (PL) properties were dramatically improved by selecting appropriately the pH value of the precursor solution; the PL intensity in the ZnSe QDs prepared at pH 6 was strongest irrespective of the size of ZnSe QDs. The XRD pattern demonstrated the formation of a cubic zinc blende ZnSe lattice structure and crystallinity.