Mathematics is a collection of cognitive products that have unique characteristics from other scientific disciplines. As cognitive products, mathematics, and thought processes are two things that cannot be separated. Although in the literature there have been many approaches proposed to support the analysis of students' thinking processes, not many have used Harel's theory of thinking to reveal the characteristics of students' mathematical thinking. Therefore, this research aims to describe the thinking process of class IX students in solving algebra problems based on Harel's thinking characteristics. To achieve this goal, qualitative research was conducted with a case study design. The questions on the topic of algebra were adapted and developed from questions from the National Examination (UN) and the International Program for Student Assessment (PISA). Next, a test was given to 30 class IX students followed by interviews. The results of the data analysis concluded that in general students still use non-referential symbolic thinking and only a few have algebraic invariance thinking or proportional and deductive thinking. This shows that there are still many students who have difficulty understanding algebra. In increasingly complex problems, students' thinking processes become less flexible, and concepts are understood separately (procedural).