The objectives of this research were to study and analyze the characteristics of computational thinking about the estimation of the students in mathematics classroom applying lesson study and open approach. Members of target group included 4th grade students of 2011 academic year of Choomchon Banchonnabot School. The Lesson plan used for data collection in this research was developed according to lesson study and open approach on the subject of estimation, which included Activity no.1 "How many notebooks would we get?" and Activity no.2 "How many trains would be needed?" Results of the research showed that the students were able to present their characteristics of computational thinking about the estimation according to the 4 steps of open approach as well as to obtain a theoretical framework of computational thinking levels about the estimation of the students in mathematics classroom applying lesson study and open approach. Considering the thinking levels of the 4th grade students of 2011 academic year of Choomchon Banchonnabot school, it showed that: the characteristics of computational thinking about the estimation of the students in mathematics classroom applying lesson study and open approach had included all levels of theoretical framework of thinking levels about the estimation of the students in mathematics classroom with lesson study and open approach, and that the thinking process about the estimation of the students in mathematics classroom using lesson study and open approach would use the reformulation of estimation before computation and estimation after computation.