This study aims to discover the effectiveness of the blended learning model in mathematics learning to improve the achievement of mathematical concepts. This study employed a quasi-experimental design with a non-equivalent control group. The experimental class was taught through blended learning, while the control class was taught through the conventional learning model. The population of this study was the students at State Senior High School 1 Sewon Yogyakarta, Indonesia. The research sample involved 60 tenth-grade students in the 2019/2020 academic year, 30 students in each of the experimental and control groups. The sampling technique used cluster sampling and the data were collected using a test instrument. The validity of the test was measured using the obtained Aiken index (0.80), and the reliability was based on the Cronbach's alpha coefficient (0.86). The data were analyzed using t-tests and descriptive statistics. The results of this study showed that the blended learning model is significantly more effective than the conventional learning model in enhancing students' achievement of mathematical concepts. This means that the application of blended learning in mathematics learning can significantly improve students' achievement of mathematical concepts.