The effect of temperature on GaAs nanowires (NWs) growth is investigated. Three samples are grown on GaAs (111) B substrate by Au-assisted metalorganic chemical vapor deposition (MOCVD) using vapor-liquid-solid (VLS) mechanism at 500 ℃, 530 ℃ and 560 ℃, respectively. It is found that all samples are vertical to the substrate. Their length growth at low temperature are independent with their diameters, and the NWs are straight from base to top. The growth rate of thick NWs increase reversely with growth temperature. That is because the VLS mechanism growth is suppressed at high temperature. For thin NWs, the growth is induced by Ga atom diffusion at high temperature. The NWs growth at high temperature taperes. That ascribes to lateral growth on NWs sidewalls. It is revealed that crystalline structure of NWs grown at low temperature is pure zinc blende (ZB) and there is no defect in thick NWs. At high temperature, there are many defects in NWs, such as stacking faults and twins.