The purpose of this research is to examine the effects of activities based on “Flipped Learning” approach on students' academic achievement and attitudes toward mathematics in mathematics lessons. A mixed method approach is used in this study aiming at investigating the effects of activities in relation with Pythagorean Relation in learning domain entitled as Geometry and Measurement in 8’th grade Mathematics lessons based on “Flipped Learning” approach, over the students' academic achievement and attitudes. Experimental design with pretest posttest control group was used in the quantitative dimension of the study. Quantitative data were collected through the academic achievement test developed by the researchers and the Mathematical Attitude Scale developed by İnan (2014) in this mixed method study aiming to examine the effects of activities based on “Flipped Learning” approach on the academic achievements and attitudes of the students in the mathematics course. The qualitative data of the study were obtained from the semi-structured interview form developed by the researchers and from the learning logs of the mathematics lessons that the students kept during the activities. In the analysis of quantitative data of the study, SPSS package program was used to calculate and analyze arithmetic mean, standard deviation and t-test. In the analysis of qualitative data, content analysis was used. The semi-structured interview form, as a data collection tool used in the qualitative data of the study, and the data obtained from the students' logs were analyzed through content analysis. It is found that the activities based on “Flipped Learning” approach positively affect students' academic achievement and attitudes toward mathematics in mathematics lessons.