Khao Yai National Park is a national park in Isaan, Thailand. Measuring 2,168 km squared, Khao Yai is the second largest national park in Thailand. In 2005 it was designated a UNESCO World Heritage Site as part of the larger Dong Phaya Yen–Khao Yai Forest Complex . Situated on the southwestern boundary of the Khorat Plateau, it occupies the western part of the Sankamphaeng Mountain range. Thick jungle covers the mountainous slopes, interspersed with some scenic waterfalls.