Some students assume that math lessons are difficult. According to (Asrori, 2009) that from various fields of study taught in schools, mathematics lessons tend to be felt difficult for students. Therefore, the need for special attention in learning mathematics students, making them successful in learning mathematics. This study aims to define students' algebraic reasoning in solving mathematical problems based on SOLO taxonomy which consists of prestructural, unistructural, multistructural, relational, and extended abstract levels according to the actual situation. The type of research used is qualitative research with descriptive method. Technique of collecting data is essay test with three item about sequence and series. The results obtained from this study that for students I in solving the problem sequence and series of items number 1, 2, and 3 in a row can be categorized at the level of unistructural, unistructural, and prestructural. Student II in solving the problem of sequence and series of items number 1, 2, and 3 respectively can be categorized at the level of prestructural, unistructural, and unistructural, while the third student in solving the problem of sequence and series of items number 1, 2 and 3 respectively can be categorized on the level of unistructural, prestructural, and prestructural.