This paper describes a study investigating students’ metacognitive behavior and abilities in the mathematic class using the open approach. Four 1st grade students, ages six to seven years, served as a target group from the primary school having participated since 2006 in the Teacher Professional Development Project with innovation of lesson study and open approach. The research was based on Begle’s conceptual framework (1969), focusing on observing the nature of occurrences in order to describe emerging facts in the class. In addition, the data were examined by triangulation among three sources: video recording, field notes, and students’ written works. Data analysis rested upon 4 open approach-based teaching steps (Inprasitha, 2010). The study results showed that the open approach-based mathematic class helped students exhibit metacognitive behavior and abilities relevant to the four teaching steps: 1) posing open-ended problem, 2) students’ self learning, 3) whole class discussion and comparison, and 4) summarization through connecting students’ mathematical ideas emerging in the classroom.