The incidence of nerve injury after endovenous thermal ablation (ETA) was investigated. Patients with incompetent great saphenous veins (GSV, 2876 legs) underwent ETA. Nerve injury, which was defined as if a patient feels any neurologic symptoms at 1 month after the surgery, was occurred in 70 legs (2.4%). The incidence was 0.8% when the ablation length was less than 40 cm, thus, 4.6% when it was over or equal to 40 cm. This finding was observed in either group patients who received radiofrequency or laser ablation therapy. Ablation conditions for below-knee GSV should be reconsidered.