This study was aimed to find out and compare the development of the predicates between
normal children and children with mental retardation of language ages(LA) 3 and 4. This is
when children's development of the predicate is very active. Out if the predicates, simple words, derivative words, and compound words were separately analyzed. Spontaneous speech of normal children and children with mental retardation of ages 3 and 4 was sampled, 10 children in each group, 40 in total. It was found that as the children grew up, the number of vocabulary list of each predicate types increased. Also, more vocabulary was used by normal children than by children with mental retardation. There are verbs and adjectives in the Korean predicate. There was a difference with derivative words when using verbs among normal children and children with mental retardation. When using adjectives, the difference was found with the number of simple words and compound words with 3-year-olds, and 4-year-olds with derivative words and compound words. Simple words were proved to be the list that divided both the normal-retardation difference and the age difference, derivative words for the comparison of normal children and children with mental retardation, and compound words for the age difference. The result of the study could be applied to be used as an evaluation criteria of the development of predicates for children with mental retardation. Furthermore, it could be implemented for pedagogical purposes.