The influx of in-migration into Bangkok Metropolis and its surrounding provinces has been recognized for a long time and tends to be more critical. The Thai government tries to alleviate this problem but so far its attempt has not yet been accomplished. Thus, it is necessary to investigate the causes of in-migration into Bangkok Metropolis and its surrounding provinces. The objectives of this study are to examine the determinants of in-migration into Bangkok Metropolis, its surrounding provinces, and Bangkok Metropolitan Area (Bangkok Metropolis and its surrounding provinces are treated as if they were a single province). Data used for this study are taken from various sources i.e. the 1980 population and Housing census conducted by the National Statistical Office, Office of the National Economic and Social Development Board, and the Department of Highway under Ministry of Transportation and Communication. The modified gravity model in the form of relative model with the multiplicative functional form is employed for studying the determinants of in-migration into Bangkok Metropolis, its surrounding provinces, and Bangkok Metropolitan Area. The findings indicate that the distance variable has the negative effects, as expected, whereas the population variable positively correlates with in-migration into Bangkok Metropolis, its surrounding provinces, and Bangkok Metropolitan Area. The relative income variable (Income j/Income i) has the positive effects on in-migration only in the case of Nakorn Pathom, Pathum Thani, Samut Prakan, and Samut Sakorn. The relative unemployment variable (UNEM i/UNEM j) positively relates with in-migration into Bangkok Metropolis, Nonthaburi, Pathum Thani, Samut Prakan, and Bangkok Metropolitan Area. The education variable in the origin provinces(LIT i) positively affects in-migration into Bangkok Metropolis, Nakorn Pathom, Samut Prakarn, and Bangkok Metropolitan Area. The relative urbanization variable (URB j/URB i) has the unexpected sign but it is not statistically significant. Based on the empirical result of this study, policies to impede or divert in-migration into Bangkok Metropolis and its surrounding provinces will not be achieved unless the government has to raise the average income level in other provinces, reduce the unemployment rates by creating more employment opportunities for rural people, modify educational curriculum to suit local occupation interests, and also lessen disparities in urbanization among provinces.