Teaching approaches in mathematics classrooms have been changed to a more insight-based problem oriented process during the last decade (van Oers, 2002). In addition, problem solving requires a variety of skills including interpreting information, planning and methodical working, checking results and trying alternative strategies (Muir, Beswick, & Williamson, 2008). The purpose of this research was to investigate students’ problem solving strategies (Posamentier & Krulik, 1998) in a problem solving- mathematics classroom, using an open approach as a teaching approach and composed of four phases: 1) posing open-ended problems, 2) students’ self learning, 3) whole class discussion, and 4) comparison, and summarization through connecting students’ mathematical ideas that emerged in the classroom (Inprasitha, 2010). Case study was employed in this study. Video and audio tape recording, and field note taking were used as methods for collecting data of a targeted group including six of grade 1 students in 2010 academic year of a school participating the Project for Professional Development of Mathematics Teachers through Lesson Study and Open Approach. Data were analyzed by using descriptive statistic and analytic description. The results revealed that the students used the problem solving strategies in all phases and mostly in the 2nd phase of the problem solving- mathematics classroom. Moreover, when the students solved the problems, they cooperated to create their problem solving strategies. Thus it could be considered that the problem solving-mathematics classroom, which starting with open-ended problems and having sessions for students to solve the problems by themselves, encourages students to create their own problems and problem solving strategies.