The purpose of this study is to investigate the acquisition of language through the mistake of inflection by Korean children of 37-84 months. In children's speech, we find morphological errors ; the incorrect insertion of epenthesis vowel ‘으’, the reconstruction of stems, the use of unstandard stems and the adding of unnecessary sound etc. Among the mistakes of inflection, the incorrect insertion of epenthesis vowel ‘으’ and misuse of reconstructed stems are closely connected with language acquisition. From a language developmental point of view, the most important period is 60 months. Until 60 months the mistakes of inflection increase, but from that time on, the errors decrease considerably.