From the sociocultural perspective, this research utilized activity theory as the theoretical framework to analyze the influences of cultural factors for Taiwanese Atayal junior high school students' study in mathematics. The research methodology adopted grounded theory, theoretical and methodological approaches which are illustrated through episodes from a study of the mathematics teaching and learning in a year-13 class, where Atayal students were considered as "lower achievers" in their year group. We show a way to address complexity in the activity of learning and its development based on recognition of central cultural factors in mathematics teaching-learning by the use of the activity system. Analysis through this systematic network can reveal the factors influencing Atayal students' learning of mathematics under their cultural background. The tension between Atayal culture's speech representation system and Chinese culture's literal representation system causes Atayal students difficulties in carrying out the transformation of representations. Tension formed between Atayal students' holistic thinking style and the analytic thinking style presented in teaching material and teaching activities is unfavorable to their understanding of mathematical concepts. From a teaching practice perspective, Atayal students can be more actively involved in mathematics learning activities when the tension between the teaching/learning systems is considered. As for the theoretical structure, the analysis structure of classroom activity and the design of teaching activities developed from a sociocultural orientation and activity theory can indeed improve Atayal students' mathematical learning within the cultural stream.