OBJECTIVE: To investigate neurophysiologic changes of peripheral nerves, which were injured by radiofrequency thermocoagulation and evaluate an effective distance between the lesioning electrode and target nerve tissue. METHOD: Thirty Sprague-Dawley rats were used and divided into three groups by the distance between the lesioning electrode and the sciatic nerve: 2 mm, 4 mm, 6 mm for each group (10 rats for each group). Radiofrequency lesioning was performed with 1.5 Volt, 1 MHz-frequency and 1 ms duration current for 90 sec. On the first and the fifth day after radiofrequency lesioning, latencies and amplitudes of compound muscle action potential were compared with the baseline values. RESULTS: No statistically significant latency change was observed on the first and the fifth day after lesioning. The amplitude was significantly reduced in group I and II on the first and the fifth day after lesioning, in contrast that, there was no significant change in the group III. CONCLUSION: There was significant decrement in the amplitude after effective radiofrequency lesioning to the sciatic nerve with the distance of 4 mm or less. However, changes of the latencies was not significant. It was suggested that effective distance between raidiofrequency lesioning electrode and target peripheral nerve was 4 mm or less.