Cadmium selenide (CdSe) nanocrystals have been synthesized in aqueous solution with Se as precursor via photo-irradiation at room temperature with the help of Na 2 SO 3 . The obtained products were analyzed through X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible absorption spectroscopy (UV-vis), and X-ray photoelectron spectroscopy (XPS). The results showed that CdSe nanoparticles with cubic phase were aggregated to an average size of 25 nm. The experiments also confirmed that Na 2 SO 3 , cetyltrimethylammonium bromide (CTAB) and trisodium citrate (TSC) played important roles in the formation of CdSe nanocrystals. The possible mechanism for the formation of CdSe nanocrystals is also proposed.