The purpose of this study was to determine differences among in-service and pre-service mathematics teachers’ opinions about teaching the thinking skills in view of their demographical characteristics. The sample of the study consists of 174 teachers and teacher candidates who are randomly selected from mathematics teachers of primary and secondary schools in Eskişehir and Eskişehir Osmangazi University Faculty of Education and master degree students without thesis. Data were collected by “Teaching the Thinking Skills Questionnaire for Mathematics Teachers” with demographical information form. ANOVA and t-test were employed to analyze the data. According to the results of the study, there were some differences among the mathematics teachers’ and teacher candidates’ opinions about teaching the thinking skills in view of their demographical characteristics.