Maternal undernutrition causes fetal growth restriction. Protein is a vital dietary nutrient for fetal growth, and branched-chain amino acids (BCAA) are noted to have anabolic actions. In this study, we investigated the effects of maternal high-protein diet or BCAA-supplemented diet upon fetal growth under the condition of maternal calorie restriction. Pregnant mice were calorie-restricted (undernutrition: UN), using either a standard diet (S-UN group), high-protein diet (HP-UN group), or BCAA-supplemented diet (BCAA-UN group) to 70% of the control; dams fed ad libitum with a standard diet (S-NN group) from 10.5days post coitum (dpc) to 18.5dpc. The fetal weights of UN groups were significantly decreased compared to that of S-NN. However, the fetal weights of HP-UN and BCAA-UN were significantly higher by 5% and 4%, respectively, than those of S-UN, concomitant with augmentation of the gene and protein expressions of IGF-I and IGF-II in fetal liver. A high-protein diet as well as BCAA-supplemented diet partially improved fetal growth restriction caused by maternal calorie-restriction, suggesting a pivotal role of them in the amelioration of fetal growth restriction.