We studied the effects of post-annealing in H2 and O2 ambients on field-emission properties of vertically-aligned ZnO nanowire arrays synthesized by carbothermal reduction process. The turn-on electric field was dramatically decreased from 3.78 to 2.37 V/microm after post-annealing in H2 ambient, which was explained by both hydrogen passivation effects of deep levels and surface modification. In other words, we could observe significant decrease of deep level peak in photoluminescence measurements on hydrogen post-annealed ZnO nanowire array. And also hydrogen-related bonds are strongly increased from X-ray photoelectron spectroscopy measurements. These findings suggest that the concentration of conduction electrons increased by hydrogen post-annealing, which results in the enhanced tunneling probability of conduction electrons into the vacuum.