Objectives: To compare the frequency of inter twin birth weight discordance, and degree of discordance rate in twin births by gestational age. Methods: There were 51,783 pairs of twins from 2007-2014 of birth certificated data of Korea Statistics. We excluded extra-marital and non-hospital birth cases. Birth weight discordance rate (%) was calculated as (birth weight of larger twin-birth weight of smaller twin)/birth weight of larger twin and multiplied with 100. Birth weight discordance values was calculated to describe the degree of discordance by 50th, 90th, and 95th percentile, and two standard deviations above the mean, and frequency of severe birth weight discordancy (birth weight discordancy equal to or more than 25%) at each gestational age. Results: The mean discordance rate for inter twin was 11.0%±9.3%. The discordance values was not a normal (Gaussian) distribution (Kolmogorov-Smirnov test: p-value=0.01), and the best-fit function was exponential model (R²=0.94, p<0.01). The cumulative frequency of discordance values had cubic model (R²=0.99, p<0.01). The 50th percentile of birth weight discordancy was 8.8% discordance, the 90th was 23.4%, and the 95th was 29.1%. At earlier gestational age, the percentage discordant according to the 95th percentile of birth weight discordance was higher (37.0% at 28-33 weeks) than at older gestational age (approximately 26.0% at ≥37 weeks). The values of 95th percentile represented approximately the severe birth weight discordancy (≥25%) in gestational age at 37 weeks and older. Conclusions: The 95th percentile corresponded to a birth weight discordancy of approximately 25 percent at older gestational age (≥37 weeks). The degree of twin births weight discordance should be categorized with respect to gestational age since the degree of discordance may vary in different gestational age.