ABSTRACTThis research were based on the lack of students’ activity and students’ understanding of mathematical concept that still low. This research aims to know the students’ activities and students’ understanding of mathematical concept  by implementing the One to One strategy in class VIII of SMPN 7 Lubuk Basung. This was an experimental research. The population was students’ class VIII of SMPN 7 Lubuk Basung. The experimental class was VIII.I and the control class was VIII.2. The Instrument of the research were observation form for the students’ activities and the final tes for the students’ understanding of mathematical concept. During the process of teaching by implementing One to One strategy, the students’ activities increase. The technique of data analysis was t-test one way and the result is  = 5,95 and = 1,69. Because > it can be concluded that students’ understanding of mathematical concept by implementing  One to One strategy was better than students’ understanding of mathematical concept by implementing conventional learning in class VIII of SMPN 7 Lubuk Basung.