Using pulsed laser ablation with arsenic over-pressure, the growth conditions for GaAs nanowires have been systematically investigated. The single-crystal structure and geometry of the nanowires have been characterized for various growth conditions. Arsenic over-pressure with As₂ molecules was introduced into the system by thermal decomposition of polycrystalline GaAs to control the stoichiometry and shape of the nanowires during growth. GaAs nanowires exhibit a variety of geometries under varying arsenic over-pressures, which can be understood by different growth processes via a vapor–liquid–solid mechanism. Without As₂ over-pressure, branched growth of GaAs with uncontrollable size and geometry was observed due to the decomposition of GaAs nanowires, producing metallic Ga which serves as catalysts for the branched growth of GaAs on the nanowire surfaces. Under optimal As₂ over-pressure, single-crystal GaAs nanowires with uniform diameter, small diameter distribution, length over 20 µm and thin surface oxide layer were obtained and used for I–V characterization.