Copper is an essential micronutrient whose deficiency is often seen to occur in humans. Although many biomedical studies have focused on the use of nanoparticles, the nutritional effects of nano-sized copper oxide particles are not well known. This aim of this study was to investigate the nutritional bioavailability of nano- and micro-sized copper oxide (CuO) particles in copper-deficient (CuD) mice. Copper deficiency was induced in mice by feeding a CuD diet (0.93 mg Cu/kg diet) for 7 weeks. After the induction of copper deficiency, nano- or micro-sized copper oxide particles were administered orally at two different doses (0.8 and 4.0 mg CuO/kg body weight) to mice in the following groups: (1) normal control (NC), (2) CuD, (3) low dose micro-sized CuO, (4) high dose micro-sized CuO, (5) low dose nano-sized CuO, and (6) high dose nano-sized CuO. The hepatic copper concentration in the CuD group was significantly lower than that in the NC group. Compared to the NC group, the CuD group exhibited lower serum ceruloplasmin (CP) activity and CP level. The copper/zinc-superoxide dismutase activity in the CuD group was significantly lower than that in the NC group. Treatment with nano- or micro-sized copper oxide particles for 2 weeks restored the hepatic copper levels and serum CP activities to values similar to those observed in the NC group. The CP levels and copper/zinc-superoxide dismutase activities in all the copper oxide treatment groups also recovered to normal values after 3 weeks of copper oxide treatment. These results show that oral administration of either nano- or micro-sized copper oxide particles for 2–3 weeks restored the normal condition in previously CuD mice.