Background: This study investigates the relationship of socioeconomic status with adverse birth outcomes (low birth weight and preterm birth) and the relationship of socioeconomic status with infant mortality, using the birth cohort in Korea, 1995-2010. Methods: In Korea, 8,648,035 births from 1995 to 2010 were studied with respect to social variation in adverse birth outcomes and infant mortality. The effect of social inequality was examined against adverse birth outcomes and infant mortality using multivariate logistic regression after controlling for other covariates. Results: Social inequality were observed in adverse birth outcomes: low birth weight (LBW, 1,500-2,499 g), very LBW (1,000-1,499 g), and extremely LBW (500-999 g) as well as moderately preterm birth (PTB, 33-36 weeks), very PTB (28-32 weeks), extremely PTB (22-27 weeks), and infant mortality. The effect of social inequality was higher among moderately LBW (1,500-2,499 g) and PTB (33-36 weeks) than very or extremely LBW and PTB. Conclusion: The social inequality in adverse birth outcomes (low birthweight and preterm) and infant mortality existed and increased in Korea from 1995 to 2010. The effect of maternal education on adverse birth outcomes as well as infant mortality was apparent in the study results. Especially, social inequality in infant mortality was greater among the sub-normal births (low birth weight [1,500-2,499 g] or preterm birth [33-36 weeks]), which suggests, social interventions should aim at more among the subnormal births. This study suggest that tackling inequality in births as well as infant mortality should be focused on the social inequality itself.