Gestational age and infant birth weight are influenced by gestational weight gain. This study was aimed to examine the effects of gestational weight gain on pregnancy outcomes. Pregnant women were recruited at two hospitals in Seoul area. Characteristics and dietary intakes of pregnant women were obtained using 24-hour recall questionnaires. Gestational weight gain was categorized as less (Under-gain) than, within (Recommended gain), or greater (Over-gain) than the Institute of Medicine guidelines. Maternal height and pre-pregnancy weight in the over-gain group significantly higher than under-gain and recommended gain group. Mini dietary assessment score of eating bean has significantly higher in under-gain group than recommended gain group and eating kimchi has significantly higher in undergain group than over-gain group. Score of eating fruit was significantly higher in over-gain group than other groups. The mean intake of carbohydrate in the recommended gain group were significantly higher than under-gain group, and mean intake of potassium in the over-gain group were significantly higher than under-gain group. Under-gain group showed the high rate of the preterm delivery and low birth weight infant delivery. However recommended gain group showed 46% reduced risk of preterm delivery (OR = 0.54 CI = 0.30-0.98). Risk of macrosomia increased with increasing gestational weight gain (p for trend < 0.05). In conclusion, pregnancy outcomes were influenced by gestational weight gain. Therefore, these finding suggested adequate gestational weight gain according to BMI for reducing the risk of preterm delivery, low birth weight and macrosomia.