Concerns about the quality of the Nations\' public schools has stimulated an unprecedented interest in educational reform. At the national level, government has launched a vigorous plan to reduce class-size to 35 from elementary to secondary schools by spending 1,700,000 billion Korean Won until 2003. For this project, this study tries to analyse and review researches about class size reduction conducted in foreign countries during last 20 years. Researchers have used various techniques to study how class size affects the quality of education. In the last 20 years, they have looked at the relationship between class size and student achievement, and have conducted various kinds of studies related to class size and its possible influence on educational practice. Researches such as Prime Time, STAR, SAGE, CSR, etc. convincing evidence that class size reduction can directly improve student learning. For instance: ◦ Smaller class size increase student achievement. ◦ Smaller classes can help lay a strong foundation in basic skills for young children. ◦ Smaller classes reduce discipline problems and increase instruction time for teachers. ◦ Smaller classes has the greatest impact on disadvantaged and retarded. ◦ Smaller classes are most effective when implemented early grades.