Objectives: To compare the secular trend (1997-2014) of gestational age specific preterm birth rate in singleton and multiple birth. Methods: The birth certificate data of Statistics Korea was used for this study (1997-98: 1,292,336 births, 2013-14: 849,779 births). The data of extra-marital birth and missing information cases (gestational age, maternal age and other variables) were excluded from all analyses. Odds ratio and 95% confidence intervals were calculated from logistic regression to describe the secular trend of very preterm birth (≤31 weeks), moderate preterm birth (32-33 weeks), late preterm birth rate (34-36 weeks) in singleton and multiple birth adjusted by maternal age (15, 20, 25, 30, 35, 40, 45), birth order (1st=1, 2nd=2, 3rd=3), infantile gender (male=1, female=0), and education (≤middle=1, high=2, college/university=3). Results: The rate of preterm birth increased 1.9 times, from 3.31% to 6.44%, during 1997-2014. After adjustment by logistic regression for infantile gender, parity and maternal age, and type of birth, the odds ratio of preterm birth of phase II was 1.69 (95% confidence interval: 1.66-1.71), compared with phase I. During the period, preterm birth rate increased 2.71 times in multiple birth, whereas the rate was 1.57 times increment in singleton birth. 47.2% of the overall increase in the preterm birth rate was attributable to the increase of preterm birth in multiple birth during the period. The odds ratio of very preterm birth, moderate preterm birth and late preterm birth rate in singleton birth for phase II were, respectively, 1.37 (95% confidence interval: 1.30-1.44), 1.08 (1.03-1.14), and 1.68 (1.65-1.72), compared with preterm birth rate of phase I. Comparing the preterm birth rate of phase I, the odds ratio of preterm birth in multiple birth of phase II was 2.61 (2.32-2.93) for very preterm birth, 1.35 (1.22-1.49) for moderate preterm birth and 2.78 (2.64-2.92) for late preterm birth rate. Conclusions: The rate of gestational age specific preterm birth increased higher in multiple birth than that of singleton birth during the period. The remainder of the total increment in the preterm birth between phase I & II was explained by increase the multiple birth and late preterm birth. There is a need for close attention in this area to understand the contributing factors to late preterm birth and to reduce preterm birth rate for multiple birth.