One of Thailand's most valuable nature preserves, Nam Nao National Park covers 966 sq km across the Phetchabun Mountains of Chaiyaphum and Phetchabun Provinces, just beyond Khon Kaen Province. Although it covers remote territory (this remained a People's Liberation Army of Thailand stronghold until the early 1980s), Hwy 12 makes access easy.
With an average elevation of 800m, temperatures are fairly cool year-round (nám nŏw means 'the water feels cold') and frost can occur in December and January. There are both evergreen and deciduous forest mixed with some vast bamboo groves. Nam Nao lies at the heart of the Western Isaan Forest Complex, a 6000-sq-km block of eight connected preserves (including Phu Kradueng National Park and Phu Khiaw Wildlife Sanctuary; permits much be acquired in advance to visit the latter), so wildlife is particularly abundant. Elephant encounters are common enough that there's an electric fence around the campground. Lucky visitors might also spot Malayan sun bears, banteng (wild cattle), Asian jackals, barking deer, gibbons, pangolins and flying squirrels. There are even a few tigers. More than 200 species of bird, including great hornbill and silver pheasant, fly through the forest, and the exceptional visibility makes this one of Thailand's best birdwatching sites.
A fair system of hiking trails branches out from the visitor centre to several scenic overlooks. Haewsai Waterfall is 17km east of the visitor centre, while to the west lie the the best sunrise/morning fog (5km) and sunset (11km) viewpoints. When the park is busy, vehicles (70B per person) deliver people to the viewpoints; reserve a seat at the visitor centre.
There are campsites, a variety of bungalows and some simple restaurants around the visitor centre.
Most buses heading west to Lom Sak or Phitsanulok from the ordinary bus terminal in Khon Kaen (120B, 2½ hours) will stop in the park. The visitor centre is 1.5km from the highway.