This study examined the structure of preschool children's natural categories by typicality ratings and category membership decisions. Half of 50 5-and 6-year-old children made typicality ratings (3-point scale) as the other half made category membership decisions for the 20 candidate examplars of each of 4 categories: “animal”,“vehicle”,“insect” and “food”. The results were as follows. There were continuous degrees of typicality ratings in two categories. Between-subjects disagreement of category membership decisions was high at intermediate typicality levels, indicating that the category boundary was unclear and fuzzy. Factor analysis and clustering suggested that category extensions of children were different from those of adults.