PURPOSE: To figure out the contribution of maternal age distribution to the preterm birth (PTB) rate of multiple births between 1997~98 and 2014~15. METHODS: Multiple birth certificate data of Korea Statistics were used for this analysis. There were 18,557 births in 1997~98 and 30,992 births in 2014~15. Kitagawa's decomposition method was used to examine the contribution of age-specific PTB rate and maternal age distribution of multiple births to overall increment of PTB rate in multiple births between 1997~98 and 2014~15. RESULTS: PTB rate of multiple births increased from 32.40 percent to 58.22 percent (odds ratio: 1.80, 95% confidence interval: 1.76~1.84) during 1997-2015. PTB rate of multiple births greatly increased for women aged 25~29 years (odds ratio: 2.09) during the same period. The rates increased 1.88 times for women aged ≤24 years, followed by women aged 30~34 years (OR: 1.65), women aged 35~39 years (1.54), and women aged ≥ 40 years (1.36). Most (78.7%) of the overall increment in PTB rate of multiple births was attributable to the increase in the dimension of women aged 30~34 years, and 49.9 percent for women aged 35~39 years. CONCLUSION: The total increment in the PTB rate of multiple births was explained by increase the proportion and the PTB rate of women aged 30~39 years. More research is needed to comprehend the contributing age factors to PTB rate of multiple births.