The objectives of this study are 1) to study the level and tempo of urbanization in Thailand during 1960-2000, 2) to study the component of urban population growth, and 3) to study the relationship between the level of urbanization and level of economic development during this period. The data used are from reports of Population and Housing Census in 1960-2000, Survey of Population Change during the intercensal period and World Population Prospects. Additionally, micro data from the 1960-2000 censuses and databases from International Monetary Fund and Food and Agricultural Organization are utilized. The indicators adopted are 1) indicators for urbanization: percent of population in urban area and urban-rural growth difference rate and 2) socioeconomic development indicators: GDP per capita, percent GDP from the non-agricultural sector and percent non-agricultural labour force. The results show that the level and tempo of urbanization varied substantially during 1960-2000 due to the population changes in urban and rural areas. Controlling for area and population changes in Bangkok Metropolis, the reclassification from rural to urban areas helps reduce the degree of primate city of Bangkok, and consequently affects the level and tempo of urbanization in Thailand. Moreover, it helped increase the level of urbanization in Thailand. Regarding the component of urban population growth during the period of study, it is found that the natural increase plays an important role. However, it becomes less important over time. The net urban-rural migration has very little impact on the urban population growth. The area reclassification has the greatest impact on urban population growth in the later period. Regarding the population growth in Bangkok Metropolis, natural increase contributes mostly during 1960-2000. The exception is for 1970-1980 when annexation of Bangkok and Thonburi is the most important factor affecting the population growth in Bangkok Metropolis. This is because Bangkok and Thonburi municipalities with the annexation of their non-municipalities were combined and upgraded to Bangkok Metropolis. Regarding the relationship between level of urbanization and level of economic development, it is found that Thailand has experienced under urbanization when its situation is compared with several countries in the world. The suitable level of urbanization to the level of economic development in Thailand is proposed for further study.