Semiconductor CdS nanowire arrays have been synthesized by the electrochemical deposition from aqueous solutions into porous anodic alumina substrates. X-ray diffraction analyses show that the as-synthesized nanowires have a highly preferential orientation. Scanning electron microscopy indicate that high-filling, ordered, and single-crystalline nanowire arrays have been obtained. The optical absorption spectra of the nanowire arrays show that the optical absorption band edge of the CdS nanowire array exhibits a blue shift compared with that of bulk CdS. The growth mechanism and the electrochemical deposition process are discussed together with the chemical compositions analysis.