Onomatopoeia expression is a sensitive word or a grouping of words that imitates the sound it is describing, suggesting its source object. This research report investigates onomatopoeia expression by preschool children and their teachers during activities in a natural setting. This report also considers the relationship between the onomatopoeia expression and the natural setting. The investigation continued from April 18 to July 18, 2007. The subjects were children 3-5 years old and their classroom teachers. To capture onomatopoeia data in a natural setting, children and their teachers were observed by investigators through video and audio recordings. The following three discussions were obtained. 1) The onomatopoeia expression develops over early childhood. 2) Onomatopoeia expression can be an effective way to describe the immediate environment. 3) Teachers can help children to understand the outside world by utilizing onomatopoeia expressions and encouraging their use by children in describing surrounding things or phenomenon. These three points will continue to be studied in future investigations.