The use of blackboard is not merely for displaying essential teachers’ information, but also encouraging
students’ active participations (Ninomiya, 2010). This study aimed to explore the use of blackboard by a teacher to promote students’ mathematical connections in a classroom of 4th grade students implementing lesson study and open approach. The open approach that applied in this study has 4 steps including 1) posing open-ended problem situation 2) student’s self-learning 3) whole class discussion and comparison 4) summarization through connecting students’ mathematical ideas (Inprasitha, 2010). The lesson study cycle involved with 3 phases : 1) Research lesson planning 2) Research lesson implementation 3) Post-research lesson discussion and reflection (Inprasitha, 2006).