Well-aligned small-diameter ZnO nanowire arrays have been synthesized on Si wafers pre-coated with c-oriented ZnO thin films by a catalyst-free thermal evaporation method. The morphology of the products has been found to be greatly affected by the oxygen flow rate. The self-catalyzed VLS mechanism is proposed to interpret the growth of the ZnO nanowires and the change in the product morphology. Classical nucleation theory is employed to analyze the growth process of the ZnO nanowires, and Zn vapor supersaturation is proposed to be a key factor to affect the diameter and the areal density of the ZnO nanowires. The average diameter of the ZnO nanowires can be finely controlled in the range of 12–31 nm by controlling the oxygen flow rate and hence the Zn vapor supersaturation. This result is consistent with our theoretical prediction. X-ray photoelectron spectroscopy measurements were performed to characterize the stoichiometry of nanowires with various average diameters. It also revealed that there are many OH species on the nanowire surface. Photoluminescence measurements showed that the deep-level emission of the nanowires with average diameter of 12 nm peaks in the yellow region, which may be attributed to the presence of OH species and surface effects.