Examples of demographic transition include transitions from high-fertility-high-mortality to high-fertility-low-mortality and to low-fertility-low-mortality. In general, a high population growth rate is observed in the high-fertility-low-mortality phase, and the rate of population growth decreases in the low-fertility-low-mortality phase. Using available demographic data from the Kingdom of Tonga for the years 1891 to 2011, we described the demographic transition pattern in this country. Since 1953, the crude mortality rate has been lower than 10‰, while the crude birth rate remained as high as 27‰ until 2011. Despite the high fertility and low mortality rates from 1996 to 2006, the mean annual population growth rate was only 4.2‰, which is attributable to the net migration rate of-17.8‰. In addition, out-migration of both young and older adults, together with the high fertility rate, contributed to the maintenance of the pyramidal shape of the population age structure of the country from 1956 to 2006. This study shows that this MIRAB (migration, remittance, aid financed, and bureaucracy) society, has been experiencing a unique demographic transition due to a high out-migration rate. Because the international migration rate has been increasing in various regions throughout the world, we may need to re-examine the demographic transition theory while considering the significant effects of international migration.