Analysis of a large, nationally representative survey shows that family size exerts a substantial negative influence on the probability that a child will attend secondary school in Thailand. The primary mechanism underlying this effect is most likely the dilution of familial resources available per child associated with larger numbers of children. The extent and the level of schooling at which this effect operates vary with the household level of wealth and with rural or urban residence. Because fertility decline is leading to a major increase in the proportion of children who come from small families, falling birth rates contribute to increasing educational attainment in Thailand.