Abstract: Objective To evaluate effects of 2.5 mg and 5 mg of dexamethasone combined with ropivacaine for ultrasound-guided brachial plexus nerve block. Methods A total of 90 patients undergoing humerus surgery were enrolled in this study from May 2018 to July 2019, and patients were randomly divided into three groups, namely group D2.5, D5 and C , respectively. Patients received ultrasound-guided supraclavicular nerve block with dexamethasone 2.5 mg (group D2.5), 5 mg (group D5) or just saline combined with 150 mg ropivacaine (group C). The onset time of sensory and motor block and the time of the first analgesic administration after surgery (sensory block time) and the duration of motor block were recorded. Dosages of parecoxib sodium and oxycodone hydrochloride prescribed 48 hours after surgery together with rest and dynamic numeric rating scale (NRS) at 12, 24 and 48 hours after surgery were recorded. Results There were no significant differences in the onset time of sensory and motor block between the three groups. In group D2.5 and group D5, the time of first analgesic administration and the duration of motor block were significantly longer than those of group C (all P＜0.01). The dosages of parecoxib sodium in group D2.5 and oxycodone hydrochloride in group D5 within 48 hours after surgery were lower than those of group C (all P＜0.05). At 12 h after surgery, the rest and dynamic NRS were significantly lower in the group D2.5 and group D5 than those in the group C (P＜0.01). Postoperative blood glucose levels were significantly higher in group D2.5 and group D5 than those of group C (all P＜0.01). The blood glucose levels after operation were significantly higher than those before operation in group D2.5 and group D5 (P＜0.01). Conclusion Dexamethasone 2.5 mg or 5 mg are equivalent in enhancing brachial plexus block of single-shot supraclavicular brachial plexus block with ropivacaine and the time of the first analgesic administration after surgery.