Arrays of single-crystal zinc oxide (ZnO) nanowires have been synthesized on silicon substrates by vapor-liquid-solid growth techniques. The effects of growth conditions including substrate temperature and Ar gas flow rate on the growth properties of ZnO nanowire arrays were studied. Structural and optical characterization was performed using scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. SEM images of the ZnO nanowire arrays grown at various Ar gas flow rates indicated that the alignment and structural features of ZnO nanowires were affected by the gas flow rate. The PL of the ZnO nanowire arrays exhibited a strong ultraviolet (UV) emission at 380 nm and a weak green emission at around 510 nm. A blue shift and broadening of the UV emission were observed with an increase in Ar gas flow rate.