A total of 97 species of flies collected in Korea since 1919 is described which consists of the collections by Kobayashi, Hori and the author. They include 19 species belonging to 12 genera of the family Calliphoridae, 38 species belonging to 15 genera of the family Sarcophagidae and 40 species belonging to 14 genera of the family Muscidae. The specimens were identified according to the male genitalia, which generally show little variations among the flies of the same species, and were illustrated. The geographical distribution and localities in Korea of all the species are presented, and the medically important species are discussed in detail. Of these flies, the calliphorid species in Korea are described and illustrated in this paper. The author found newly 8 species of calliphorid flies including a new species. As for the families Sarcophagidae and Muscidae, the descriptions on these will be published in part II and part III, respectively, in the near future.