The present study examined the knowledge of onomatopoeia in 4- to 6-year-olds. The participants undertook two onomatopoeia tests (a test of onomatopoeia related to action/state, and emotion/sense). Each onomatopoeia test contained four categories: the onomatopoeia of action/state (i.e., "eating and drinking", "walking", "talking", and "seeing") and the onomatopoeia of emotion/sense (i.e., "laughing", "crying", "thinking and feeling", and "aching"). Each category had four onomatopoeia words. The participants were presented with four sets of pictures that described four onomatopoeia words under two conditions: a within category condition in which four pictures were selected from just one of the four categories, and a between category condition in which four pictures were selected from each of the four categories respectively. The participants were asked to choose one picture for each onomatopoeia word from the four alternatives. We found: 1) Young children's onomatopoeia knowledge underwent marked improvement from 4 to 5 years of age. 2) In the action/state test, a between category condition was more accurate than a within category condition, whereas in the emotion/sense test there was no difference between two conditions. 3) According to the onomatopoeia words, young children's understanding levels of those are different. These results are discussed from the perspective of the level of young children's knowledge of onomatopoeia.