The purpose of this study is to examine meaning, factors, and diagnostic method of underachievers in mathematics and to propose the idea of the individualized teaching method for underachievers. For the first purpose, previous literatures on underachievers are reviewed. For the second purpose, this study described the procedure of developing the individualized instructional program, and the method of diagnosing and prescribing errors in mathematics. In the future, individualized instructional program need to be developed based on correct diagnosis of factors effecting under achievement for the effective teaching of underachievers.