Khao Yai National Park is a national park in Thailand. It lies largely in Nakhon Ratchasima Province, but also includes parts of Saraburi, Prachinburi and Nakhon Nayok provinces. It was the country s first national park, established on September 18, 1962. A major role in its establishment was done by Boonsong Lekakul, one of the 20th century is most famous conservationists in Thailand.