This quasi experimental design aims to know the effectiveness of problem posing approach in towards student’s mathematical conceptual understanding skill. Problem posing approach is an approach in mathematics learning which students are asked to formulate, form, and ask some questions or problems by creating some similar problems to the questions given by the teacher or to the situation provided. The research design is posttets only control group design and the population is the students of 8th grade even semester of SMPN 22 Bandar  Lampung academic year 2012/2013 and the sample of this research are students in class VIII-H and VIII-G that are determined by purposive sampling technique. The kind of data of this research is quantitative which it is in the form of student’s mathematical conceptual understanding score that is aqcuired by student’s mathematical conceptual understanding test.  Based on research results, it is concluded that problem posing approach is effective in towards student’s mathematical conceptual understanding skills (in circle concepts), a case study in 8th grade students of even semester Junior High School State 22 Bandarlampung in Academic Years 2012/2013.Keywords:  Effectiveness, Problem Posing, Conceptual Understanding