Background: Cuminum cyminum fruit essential oil (FEO) dose-dependently can attenuate the expression of morphine tolerance and dependence in morphine-dependent mice. Objective: In this study, the effects of Cuminum cyminum FEO on acquisition and expression of morphine-induced conditioned place preference (CPP) in morphine-sensitized mice were studied. Methods: Repeated subcutaneous (s.c.) administration of morphine (5 mg/kg), once daily for three and 5 days free of the opioid (sensitization period), increased conditioning response induced by ineffective doses of morphine (0.25, 0.5 and 0.75 mg/kg). Results: The results showed that intra-peritoneal (i.p.) injection of Cumin FEO (0.001, 0.01, 0.1, 0.5, 1 and 2% 5 ml/kg) or Tween-80 (0.5% 5 ml/kg), 60 min before administration of morphine or saline during sensitization period (acquisition), decreased the conditioning response induced by ineffective dose of morphine (0.5 mg/kg s.c.) at the doses of 1% (P<0.05) and 2% (P<0.001) while Cumin FEO (0.001-2% i.p.), just 60 min before the test on post-conditioning phase (expression experiments), did not alter the conditioning scores in morphine- and non-sensitized mice. Conclusion: Our findings showed that the Cuminum cyminum fruit essential oil reduces the acquisition but not expression of morphine-induced conditioned place preference in morphine-sensitized mice.