Early to Middle Permian ostracods of the Indochina block were reported from Central Thailand. Twelve Permian localities spreading in the Loei, Phetchabun, and Nakhon Sawan-Lopburi areas were investigated and limestone samples were collected. Out of 135 samples collected, 82 yielded ostracods. In this paper, 39 species of 20 genera belonging to Order Palaeocopida, families Aparchitidae, Kloedenellidae, Knoxitidae, Paraparchitidae, Kirkbyidae, Amphissitidae, Youngiellidae, Hollinellidae, and Coelonellidae are identified; species of Order Podocopida will be described in a second paper. Eight species are newly described: Langdaia meesooki Chitnarin n. sp., Sargentina chantarameei Chitnarin n. sp., Geffenina posterodorsospina Chitnarin n. sp., Geffenina mariebeatriceae Chitnarin n. sp., Samarella sonei Chitnarin n. sp., Samarella viscusforma Chitnarin n. sp., Microcoelonella takliensis Chitnarin n. sp., and Microcoelonella takfaensis Chitnarin n. sp. This paper presents the first intensive systematics treatment of Permian ostracods of the Indochina block. Detailed analysis of the fauna recovered Palaeocopida and Podocopida, facilitates and will contribute to a palaeoenvironmental and palaeobiogeographical reconstruction.