Objective To investigate the monitoring of the recurrent laryngeal nerve (RLN)function during thyroid surgery by recording the compound muscle action potential (CMAP) of lateral cricoarytenoid muscle(LCA) under block anaesthesia of the cervical plexus. Methods Nicolet Endeavor CR(Viasys Healthcare, USA) was applied for the intraoperative monitoring. A pair of needle electrodes were inserted into LCA to record CMAP, and a concentric electrode was used to intermittently stimulate the RLN for monitoring the RLN function during operation under block anaesthesia of the cervical plexus.Results In these 28 cases 31 RLNs were exposed during operation, CMAPs appeared with consistent latency when stimulation was applied at the exposed segment. The stimulating intensity threshold ranged from 0.2 to 1.6 mA. The average value was 0.96 mA. 25 of 31 showed CMAPs with consistent latency when stimulation was provided along the possible pathway of the unexposed segment. The stimulating intensity threshold ranged from 1.3 to 3.5 mA. The average value was 2. 03 mA. 6 of 31 did not evoked CMAP even though the maximal stimulating intensity was 5 mA. When these six nerves were exposured, compound muscle action potential were evoked consistently. All of the 28 patients had normal phonation function after surgery.Conclusions It is a safe, effective and feasible method to monitor the RL N function by recording the CMAP of lateral cricoarytenoid muscle under block anaesthesia of the cervical plexus.