The purposes of this study is to find the cost-effective reduction strategies of the class size for improvement of education achievement in schools. The class size is student number per class. The related theories, the practices of Korea and foreign countries were analysed for this purpose. The reduction strategies of the class size for improvement of education achievement in schools are as follows.First, the class size have to be reduced for maximize the education achievement of students according to the priorities in school years, texts. The priorities of class size reduction are as follow; Kindergarten and 1-3 school year in elementary school, 4-6 school year in elementary school, middle school, higher school. the largest class, the lower education achievement class, the class which contains many students of poverty home have to be reduced first and foremost in the same school year.Second, the class size have to be reduced by twenty students per class progressively. Third, the class size have to be reduced accompany with middle/long time plan, achievements, of effective new teachers for small class size and basic learning skills, and development of the teachersFourth, the class size have to be reduced accompany with many strategics for improvement of education achievement. The methods of class size reduction have to be decided by the local educational administration agency/the education institution. Fifth, The accountability system must be established and operated for the effectiveness of the class size reduction program.