Objective To investigate the effect of Dexmedetomidine on HMGB1(High mobility group box 1 protein) expression in the L4-5 dorsal horns of rats underwent spinal nerve ligation. Methods Thirty male SD rats were randomly divided into three groups( n = 10 each): sham operation group(Sham group), L5 spinal nerve ligation group(SNL group) and Dexmedetomidine-treatment group(DEX group). After intrathecal catheter implantation between L4-5 vertebrae, the rats in SNL group and DEX group were underwent L5 spinal nerve ligation. The surgical procedure for the the rats in Sham group was identical except nerve ligation. Then Dexmedetomidine (2 μg/10 μl) was injected intrathecally via intrathecal catheter in the rats of DEX group after L5 spinal nerve ligation; the same volume of normal saline was injected intraperitoneally in rats of Sham group and SNL group. Expression of HMGB1 in the spinal dorsal horn was measured by Western blot. Results Comparing to Sham group, the paw withdrawal threshold(PWT) and paw withdrawal latency(PWL) of rats in SNL group were significantly decreased from 1 day and 3 days after SNL, respectively, and lasted for 7 days( P<0.05). On day 7 after SNL, the PWT(12.9±1.6 g g) and PWL(8.3±0.9 s) were decreased 67% and 41%, respectively( P<0.05), with the levels of HMGB1 expression increased 84%( P<0.05); Comparing to SNL group, the PWT and PWL of rats in DEX group were significantly increased( P<0.05). On day 7 after SNL, the PWT(22.3±3.2 g) and PWL(10.4±1.1 s) were increased 73% and 25%, respectively( P<0.05), with the levels of HMGB1 expression decreased 29%(P<0.05). Conclusion Increase of HMGB1 expression in spinal dorsal horn was involved in SNL-induced neuropathic pain; Dexmedetomidine injected intrathecally ameliorated SNL-induced mechanical allodynia and thermal hyperalgesia; the effect may be related to inhibition of HMGB1 expression by Dexmedetomidine.