This study analyzed the characteristics of early grade students' algebraic reasoning in the context of open approach and lesson study. The study was conducted in one first grade mathematics classroom in Thailand that has used open approach and lesson study since 2006. The results showed that first grade students' algebraic reasoning included: 1) using algebraic expressions to represent addition and posing situations for given expressions, 2) constructing and using tools to find problems more easily, 3) extending solutions to other number domains, 4) using various representations to justify thinking, and 5) reasoning about number relations. Algebraic reasoning occurred when teachers and students connected the real world, semi-concrete world, and mathematics world as oriented in Inprasitha's approach.