According to author Nguyen Phu Loc (2016), “In mathematics education, teaching mathematics includes two main activities: teaching activity and learning activity; with the approach of activity theory, we can propose several active teaching methods”. In teaching the topic of circle equation belonging to Geometry 10, teachers have to (1) design learning activities for students and (2) organize activities for students to discover knowledge. Therefore, this article aims at presenting some theoretical basis and application of activity theory in designing activities in teaching circle equation.