Abstract: This study was performed to investigate the effect of tramadol on morphine dependency and analgesia. Mice were divided into 5 groups, (1) Morphine-dependent, (2) Tramadol-dependent, (3) Morphine-dependent accompanied by saline, (4) Morphine-dependent accompanied by tramadol (50 mg kg-1) and (5) Tramadol 30 min pretreatment of naloxone in the last day in morphine-dependent mice. Hot-plate, formalin and writhing tests were applied to investigate antinociceptive effect of tramadol in different doses (12.5, 25, 50 and 100 mg kg-1). Latency time for jumping in group 4 (11.64 ±1.44 min) was less than group 1 (19.62 ±2.28 min) (p<0.05). The dose of 50 and 100 mg kg-1 of tramadol induced more tolerance in mice in hot-plate test. The most of this effect is for tramadol 100 mg kg-1 30 min after beginning the test to be controlled (p<0.05). In formalin test tramadol 50 mg kg-1 in both acute (7.17 ±2.66 min) and chronic (19.5 ±9.22 min) phases showed the most effectiveness. In writhing test the most effective dose was 50 mg kg-1 of tramadol as well. It seems tramadol can increase the depth of morphine dependence in mice. Also, tramadol antinociceptive effect in high doses can appear the comparative effect with morphine in hot-plate, formalin and writhing analgesic models.