The aim of the study reported on here was to assess the impact of differentiated instruction in terms of mathematics achievement and the attitudes of secondary school learners to reveal their views on differentiated instruction. The study was designed according to a mixed method design in which both quantitative and qualitative methods were used. The study group, which constituted the quantitative dimension of the study, consisted of 2 control groups and 1 experimental group. The Mathematics Achievement Test, Mathematics Attitude Scale and a semi-structured interview form were used as data collection tools. One-way anova and descriptive analysis techniques were applied for the analysis of the data. We concluded that differentiated instruction in mathematics courses increases secondary school learners' mathematics achievement, but has no effect on their attitudes towards mathematics.