This study examines the capacity of adults with mild intellectual disabilities in understanding similes and metaphors. The subjects of this study were 30 in total, 10 adults with mild intellectual disabilities, 10 normal children matched with the language age of Mild intellectual disability adults, and 10 normal adults. The total items are 16 similes(~처럼: 4 items, ~듯이: 4 items, ~같이: 4 items, 마치~같다: 4 items) and 8 metaphors(A is B: 4 items, B of A: 4 items). The subjects were asked to read the sentences and find the appropriate pictures. As a results, the mild intellectual disability group showed lower ability to understand the similes than the other two groups. In addition, mild intellectual disabilities adults group and normal children group showed remarkable development of the ability to understand similes at age 8. According to the analysis of the subarea of the similes, it was found that the order of ‘~처럼’, ‘~듯이’, ‘~같이’, and ‘마치~같다’ was developed. And ‘~처럼’ began to develop before 7 years of age and was completed at age 7. The mild intellectual disability group showed lower ability to understand the metaphors than the other two groups. As a result of analyzing subarea of metaphors, 'A is B' was higher than 'B of A'. In the adult group with mild intellectual disability, 'A is B' increased in understanding ability of metaphor as age increased from 7 to 9. However, 'B of A' did not show any difference in all ages. According to these results, when educating people with mild intellectual disabilities, it is necessary to improve their ability to understand similes and metaphors. Especially, it is necessary to focus on the fact that the simile understanding ability is remarkably developed at the age of 8 and the metaphor understanding ability develops at the age of 9 years. In addition, it can be suggested that when teaching about understanding of similes, it is necessary to teach in order of ‘~처럼’, ‘~ 듯이’, and ‘~같이’.