The mother was 24 years old, primipara, and had been taking carbamazepine 400mg(serum concentration $5.0-8.5{mu}g/ml$) during pregnancy without any clinical seizures. A male baby with physical malformation was delivered on week 39. The malformation is extradigit(polydactily) on X-ray of right foot and left mild hydronephrosis on ultrasonography and renal scan with radioactive material. We reported this rare case and reviewed related articles about teratogenic effect of carbamazepine, mechanism of action and prevention of teratogenesis.