On the north half of the island, this lush reserve protects 23 sq km of virgin island rainforest (evergreen monsoon forest). Because of its royal status, it's better protected than the average national park in Thailand. Tigers, Malayan sun bears, rhinos and elephants once roamed the forest here, but nowadays resident animals are limited to wild boars, monkeys, slow loris, langurs, gibbons, civets, flying foxes, cobras, pythons, squirrels and other smaller creatures. The reserve's highest point is Khao Phra (442m).
A German botanist discovered the rare 3m to 5m white-backed palm (langkow palm) in Khao Phra Thaew about 50 years ago; this fan-shaped plant is found only here and in Thailand's Khao Sok National Park. Threaded with a few hiking trails and gurgling waterfalls, the reserve can be accessed from both the east (along Rte 4027) or the west (along Hwy 402).