LESSON STUDY: PROMOTING STUDENT THINKING ON THE CONCEPT OF LEAST COMMON MULTIPLE (LCM) THROUGH REALISTIC APPROACH IN THE 4 TH GRADE OF PRIMARY MATHEMATICS TEACHING Marsigit , Atmini Dhoruri, Sugiman, Ali Mahmudi The State University of Yogyakarta, Indonesia The aim of this paper is to encapsulate, through Lesson Study, the picture of mathematical thinking that is students thinking on the concept of Lowest Common Multiple (LCM) at the 4 th Grade Students of Primary School in Indonesia. With the ground of the New School-Based Curriculum in which we difined our mathematics, we, in collaboration with teacher, prepared teaching learning of LCM with Realistic Approach. The results of the study indicated that the characteristics of realistic approach in which the students thinking the concept of LCM were uncovered. INTRODUCTION Based on the Ministerial Decree No 22, 23, 24 year 2006, starting June 2006, Indonesia Government committed to implement the new curriculum for primary and secondary education, called KTSP “School-Based Curriculum”. This School-based curriculum combines two paradigms in which, one side, stressing on students competencies but also, on the other side, concerning students’ learning processes. The National Standard of Competencies is then to be elaborated to be the Basic-Competencies that is the minimum competencies that should be performed by the students, covering affective, cognitive and psychomotor competencies. Indonesian Government has developed Contextual teaching and learning (CTL) and Realistic to support School-Based Curriculum implementations; it means that the government encourages the teachers to develop students life skills by employing optimally the environment to support students’ activities. The scope of primary mathematics: Numbers, Geometry and Measurements, and Analysing Data In the National Standard for the content of our New School-Based Mathematics Curriculum for Primary School, it was stated that mathematics is the universal science and it is the ground of other sciences. Currently fascinating achievement in information technology and other subjects are the effects of good achievements in mathematics. Therefore, to gain the better future it needs good achievements in mathematics since the youth. In the Guide Line for developing School-Based Curriculum, it was stated that mathematics in primary school have its function to encourage the students to think logically, analytically, systematically, critically, creatively and be able to collaborate with others. Those competencies are needed for the students in order that they can get, access and employ information to preserve their live. Mathematical thinking is defined as students’ activities to communicate mathematical ideas in which it involves the 179