Objective To investigate the effects of different volumes of ropivacaine with the same solubility in supraclavicular brachial plexus block(SCBPB) under ultrasound guidance on ipsilateral diaphragmaticparalysis.Methods Seventy two patients (32 males,40 females, aged 18-65 years, of ASA Ⅰ or Ⅱ, scheduled for the right upper limb fracture internal fixation removal were randomized into group A (0.375% ropivacaine 20 ml) and group B (0.375% ropivacaine 30 ml), 36 cases in each group.The onset and duration of sensory and motor block were observed as well as complication.Diaphragmatic excursion were evaluated by M type ultrasound before and 30 min after drug injection under eupnea and forced respiration through observing diaphragmatic excursion to study diaphragmatic paralysis.Results The onset time of sensory block and maintaining time of sensory block or motor block had no significant difference between the two groups.Compared with group A,the onset time of motor block in group B was significantly shorter (P<0.05).Twelve cases (33.3%) in group A and 22 cases (61.1%) in group B respectively showed complete or partial hemidiaphragmatic paralysis 30 min after drug injection.Compared with group A, the rate of diaphragmatic paralysis in group B was significantly higher (P<0.05).Conclusion Supraclavicular brachial plexus block under ultrasound guidance with 0.375% ropivacaine 20 ml or 30 ml both can provide excellent anesthetic effect.0.375% ropivacaine 20 ml is less likely to lead to diaphragmatic paralysis.