This experiment was conducted to investigate the effects of dietary energy levels of gestating gilts on physiological parameters and reproductive performance for primiparous sows. A total of 40 F1 gilts (Large White${ imes}$Landrace) were allocated to 4 treatments using a completely randomized design (CRD). Four different experimental diets contained 3,165, 3,265 3,365 and 3,465 kcal of ME/kg and each diet was provided to gilts at 2.0 kg/d during gestation. Consequently, energy intake of each treatment of gestating gilts was 6,330, 6,530, 6,730 and 6,930 kcal ME/kg, respectively. During the whole gestation period, body weight, fat mass gain and backfat thickness of gilts were increased in proportion to dietary energy levels (p<0.01). However, estimated protein mass gain of gilts was not affected by dietary energy level (p>0.10). At farrowing, the total number of pigs born per litter did not show any significant difference among treatments. However, the number of pigs born alive per litter in treatment 6,730 kcal ME/d was significantly higher than that of other treatments (p<0.05). Moreover, litter weight at birth was improved as dietary energy level was increased (p<0.05). Feed intake of sows during lactation tended to decrease as dietary energy level of gestation was increased, but litter weight gain was not affected by dietary treatment during the gestation period. Fat content in colostrum was higher as dietary energy level was increased during gestation. The concentration of blood estradiol-$17{eta}$ was increased and was higher at the first trimester of gestation in 6,730 kcal ME/d treatment compared to other treatments. These results suggested that increased dietary energy level during gestation resulted in higher body weight and backfat thickness of sows. In addition, reproductive performance of the sow, such as litter weight at farrowing and the number of pigs born alive, was improved when 6,730 kcal of ME/d treatment diet was provided. Consequently, the NRC (1998) recommendation of energy for gestating gilts (6,015 to 6,150 kcal of ME/d) should be reevaluated to maximize reproductive performance because recent high-producing sows require much more energy to produce a large litter size and heavier piglets from the first parity.