Purpose – The main purpose of this study was to examine mathematics teachers’ abilities in developing formative assessments when teaching algebraic reasoning after utilizing two innovations: lesson study and open approach. Methodology – A single group quasi-experimental research design was employed in two mathematics classes in a school in Chaiyaphum province, Thailand. The target group was all the 11 mathematics teachers, who were pre-tested, then post-tested after the lesson study and open approach training. This was followed by their participation in setting up a lesson study group and conducting 10 lesson study cycles involving two research participants who were teaching two selected classes. The instruments were a questionnaire, the AssessToday rubric, interview protocols, 10 lesson plans, field notes, classroom observation video recordings, and students’ written work. The AssessToday rubric was the main instrument used to determine the ability level of the mathematics teachers in developing formative assessment in terms of seven components, namely learning targets, question quality, nature of questioning, selfevaluation, observation of student affect, instructional adjustment, and evidence of learning. Results – The pre-test and post-test results showed an improvement in the mathematics teachers’ understanding about formative assessment, the methods used in formative assessment, and how they could use the data from the formative assessment while they were implementing the two innovations. In addition, the AssessToday rubric showed that both mathematics teachers were progressing gradually from novice, apprentice to practitioner level in their abilities in developing formative assessment after the 10 cycles of innovation practices in all the seven components. The results imply that the mathematics teachers showed vast improvement in their abilities in developing formative assessment after they implemented the lesson study and open approach innovations in their teaching. Significance– The results contribute significantly to knowledge about the usefulness of the lesson study and open approach innovations in enhancing mathematics teachers’ abilities in developing formative assessment.