We investigated the diversity of lepidopteran larvae and their food plants in a 0.1 ha plot at Mt. Jirisan National Park, South Korea. The plant species in the plot were identified, and the lepidopteran larvae that fed on each plant in the plot were collected. Thus, we collected 141 plants of 16 species that belonged to 14 families and 159 lepidopteran larvae of 70 species that belonged to 11 families. Oak (Quercus serrata) was found to be the most favored plant species. Among the 11 lepidopteran families, the species richness and evenness of two families, Geometridae and Noctuidae, were the highest. On the basis of this study, we could assume the interaction between food plants and caterpillars in a temperate forest. Furthermore, we could estimate the species diversity of lepidopteran caterpillars in this temperate forest.