Even though abacus calculation has been included in elementary mathematics curriculum by the 4th curriculum (1982-1988) reform and implemented in elementary classroom, it didn\'t appear in the textbook any longer. In those days, the use of abacus Arithmetic was emphasized more on calculation rather than focusing on helping calculation process. However, in these days, the mental arithmetic with abacus has been found that it helps elementary students bring about greater success in various problem solving skills, and understanding of process of calculation. The purpose of the study is to investigate the calculation process on \'mental arithmetic\' of abacus calculation with elementary students and teachers.