Abstract: We have investigated the effect of surface dangling bonds and molecular passivation on the doping of GaAs nanowires by first-principles calculations. Results show that the positively charged surface dangling bond on Ga atom is the most stable defect for both ultrathin and large size GaAs nanowires. It can form the trap centers of holes and then prefer to capture the holes from p-type doping. Thus it could obviously reduce the efficiency of the p-type doping. We also found that the NO₂ molecule is electronegative enough to capture the unpaired electrons of surface dangling bonds, which is an ideal passivation material for the Zn-doped GaAs nanowires.