The effects of oxygen (O2) reactive ion etching (RIE) on the field emission (FE) properties of aligned CuO nanowire films are investigated systematically. It is found that the FE performance of the films is largely enhanced after initial exposure to reactive oxygen ions but degrades after extended treatment. As comparison, Ar RIE is also used to treat CuO nanowires, which, however, results in the deterioration of FE properties. The enhanced FE after O2 RIE is attributed to the shaper morphology, cleaner surface and better conductivity. On the other hand, increased work function and non-crystallized surface structure cause the deterioration of FE of CuO nanowires after Ar RIE treatments.