This study attempted to explore whether Korean children could understand the ontological distinction between the mental entities and physical entities through two studies. We used eighty 3-, 5-, 7-, 9-, and 11-year-old children(16 for each age group). In the first study, children were asked to judge the properties about the mental entities as well as the physical entities according to four different kinds of criteria such as behavioral-sensory contact, public existence, consistent existence, and having a weight. The results showed that the children older than 5-years could successfully understand the ontological distinction between the mental and physical entities, even though 3-year-olds performed around the chance level. This implied a developmental change between 3- and 5-year-olds. In the second study, children were asked to classify two stimuli according to 5 different kinds of criteria such as behavioral-sensory contact, consistent existence, occupying physical spaces, having a weight, and having an effect on other materials. Unlike study 1, we found out the developmental change between 5- and 7-year-olds: the children older than 7-years could successfully classify the stimuli, while the children younger than 5-year-olds performed below chance. As for the development of knowledge, children had more sophisticated knowledge about physical entities by the formal education, whereas intuitively understanding the mental entities.