Introduction: Our previous study has showed that chronic administration of magnesium oxide nanoparticles (MgO NP) can reduce anxiety in adult male rat. In this study the effects of MgO NP on anxiety induced by morphine withdrawal were investigated in adult male mice. Methods: Adult male NMRI mice (weighing 27 ± 3 g) divided into groups: control, receiving intraperitoneal (i.p.) injection of MgO NP (1, 2.5, 5 mg/kg), morphine withdrawal groups that receiving saline or MgO NP (2.5, 5 &10 mg/kg) as acute (a single injection at the test day) and chronic (co-injected with morphine for 4 days). To develop morphine dependency, increasing doses of morphine (20, 40, 80 mg/kg( injected subcutaneously for 4 days. Mice received a final morphine injection (40 mg/kg) 3 hours prior to naloxone (5 mg/kg (i.p.) on the day of testing (day 4). In addicted groups, after naloxone injection, morphine withdrawal signs were evaluated. In all groups, anxiety like behavior was assessed by the elevated plus maze apparatus. Results: MgO NP (2.5 & 5 mg/kg) reduced anxiety like behavior (P<0.05). Acute and chronic MgO NP injections (5&10 mg/kg) could significantly improve/alleviate anxiety like behavior (p<0.05 & p<0.01 respectively) and reduce locomotor activity (p<0.05, acute; p<0.05, & p<0.01, chronic), rearing, climbing and weight loss in morphine withdrawn mice. Conclusion: Due to the positive effect of MgO NP on anxiety like behavior and morphine withdrawal signs and symptoms, this nanoparticle can be a potential candidate for reducing the side effects of chronic usage of morphine and morphine withdrawal.