The placenta produces several growth factors, including placenta growth factor (PlGF), which are essential for placenta growth and fetal growth. Diabetic pregnancy induces the abnormal placental growth and fetal development. This study investigated whether diabetes in pregnant rats induces changes in PlGF expression in the placenta. Diabetes was induced by a single intravenous injection of streptozotocin (35 mg/kg body weight) on day 0 of pregnancy, blood and tissue samples were collected on day 20 of pregnancy. In the diabetic group, maternal body weight and fetal weight significantly decreased compared to controls. RT-PCR and Western blot analyses showed that expression of PlGF was significantly decreased in placenta by streptozotocin treatment. Immunohistochemical study showed that the positive signal of PlGF in trophoblast cells was decreased in the diabetic group compared to controls. These findings demonstrate the decline of PlGF in the placenta in diabetic pregnancy.