This paper examines the effect of universal pre-kindergarten (pre-k) on the labor force participation of mothers with pre-k-aged children in Oklahoma and Georgia. I apply the synthetic control method (SCM) to Current Population Survey (CPS) data to identify the causal relationship between universal pre-k and female labor market outcomes. I find that the universal pre-k policy has a positive impact on the intensive margin of the labor supply of mothers with pre-k-aged children in Georgia, which provides full-day child care services for all pre-k programs. However, Oklahoma's universal pre-k policy has little effect on the labor outcomes of mothers with 4-year-old children. The empirical results also suggest that universal pre-k has heterogeneous impacts on subsamples stratified by education level, marital status, poverty status, and the age structure of children in the household. (author abstract)