This study explores the effect of a history-based course on pre-service teachers' mathematical knowledge for teaching (MKT). Data were collected from 10 pre-service teachers at a normal university in China. At the beginning of the course, participants were asked to submit their teaching plans followed by a teaching simulation on selected topics. They then received a history-based course particularly stressing the value of the history of mathematics in education. After being educated on the historical background of these topics, the participants were asked to think back while watching their simulation videos and then redesign their instructional plans on the same topics. We investigated these pre-service teachers' MKT through the simulated teaching videos, conducting semi-structured interviews, and analyzing their teaching plans and learning journals. Results suggest that these pre-service teachers' MKT had improved. Their changes in PCK were more significant than those in SMK. Nevertheless, the history-based course seemed to have had less influence on the participants' CCK and KCC. Possible reasons for the positive and negative effects are addressed.