This study describes secondary school mathematics teachers' use of history of mathematics in their classes and their knowledge levels in this field. The study population included a total of 58 secondary school mathematics teachers working at the secondary schools located in Yozgat city center, and the sample included 32 mathematics teachers from 10 secondary schools who volunteered to take place in the study. The data collection tools used in the study were an 11-item short-answer history of mathematics knowledge level test and a 5-item written opinion form in order to identify teachers' history of mathematics use. The findings showed that most teachers did not use history of mathematics in their classes. The findings also showed that teachers who used history of mathematics did so by mentioning the life stories of mathematicians and the contributions of ancient civilizations to mathematics in order to motivate students at the beginning of their classes. The mean history of mathematics knowledge test score of the teachers who participated in the study was 3.5. This finding shows that the teachers in general had low history of mathematics knowledge levels. Also it was found that those who did not use history of mathematics in their classes scored lower in the history of mathematics knowledge test than those who used it in their classes.