Objective To explore the significance of intra carotid arterial diazepam injection in lateralization of intractable epilepsy with bilateral synchronous epileptiform discharges. Methods Intra carotid arterial diazepam injection test was performed in 20 patients suffering from intractable epilepsy with bilateral synchronous epileptiform discharges under EEG monitoring. Side of the epileptogenic focus was determined by observation of the inhibition of epileptiform discharges after injection. Results After diazepam injection, disappearance of bilateral synchronous epileptiform discharges was found in 16 patients (80%), so the injection side was decided as the epilepogenic focus. Disappearance of the epileptiform discharges was found at the injection side, but disappearance of the bilateral epileptiform discharges was found after injection at the opposite side in 4 cases (20%). The accuracy of lateralization was proved by surgery, electrocorticography (EcoG), and depth electroencephalography (DEEG) in 12 out of the 20 cases. During the follow up for 1-6 5 years after surgery and gamma knife treatment, disappearance of clinical seizure and epileptiform discharges of EEG and improvement were found in 15 cases (75%) and 5 cases (25%), respectively. Conclusion Intra carotid arterial diazepam injection is of important clinical value in lateralization of the epileptogenic focus before surgery for patients with intractable epilepsy.