From 1990 to 2000, it is revealed that population growth in the Hmong and Karen hilltribe minorities in the northern part of Thailand was reduced. The growth was not in line with the increase in their socio-economic development. Socio-economic changes were slower for the Karen, whereas economic growth was lower for the Hmong. The Hmong had more interaction with Thai society than the Karen. However, it does not mean that the Hmong accepted birth control and family planning policies of the Thai government. It is found that educational changes, jobs and better economic status were significant to population growth. Hmongs living in urban areas had more children than those living in rural areas. The finding is not surprising as the ethnic group tends to have more children, particularly sons, which affected the population growth rates of the Hmong.