The aim of the study is to investigate the relationship between the teaching approach adopted by mathematics teachers and their 9th grade students' mathematical self. The study searched for the answers on three research questions, namely, about 1) the approaches prevailing in mathematics teachers' beliefs about effective teaching and self-reports about their classroom practices, 2) the qualitative and quantitative features of students' mathematical self, and 3) the relationships between the teaching approaches supported by mathematics teachers, the indicators of their 9th grade students' mathematical self, teachers' socio-demographic indicators, and students' socio-demographic indicators. The outcomes of the study show that because of the complex structure of the phenomena, it is difficult to classify mathematics teachers' beliefs on teaching and their self-reported practice into theoretically predefined groups though the use of constructivism in a lesson has a more positive influence on students' mathematical self than mere support of the constructivist beliefs.