Stretching over approximately 2,168 square kilometers in the east, Khao Yai National Park claims to be the second largest national park in Thailand. In 2005, it was declared a UNESCO World Heritage Site. It spreads over 4 provinces namely Nakhon Nayok, Prachin Buri, Nakhon Ratchasima and Saraburi. The park is also a designated ASEAN Heritage Park since 1984. It was established on September 18, 1962 and was accredited as the first national park in Thailand royal proclamation in the Government Gazette.
The National Park is situated almost 175 km northeast of Bangkok and is on the southwestern boundary of the Khorat Plateau, it occupies the western part of the Sankamphaeng Mountain range. The highest point of summit is Khao Rom which is around 1,351 meters above the sea level.
Khao Yai National Park is home to around 3,000 species of plants, 320 species of birds, and 66 species of mammals.
The national park has some of the most spectacular waterfalls of Thailand the most popular ones are Heo Narok, Pha Kluaymai, and of course the Heo Suwat made famous by the Leonardo de Caprio starrer The Beach released in 2000.
The park sees three main season (although none of it results in the park to close for visitors) (information source: Wikipedia):
Khao Yai National Park is one of the very few places in Thailand which still has wild elephants which can be seen quite frequently and forma major part of the tourist attraction. Other large animals include gaur, Asian black bear, gibbon, pig-tailed macaque, and wild pig. Some of the birds that can be spotted around the park include Chestnut-headed bee-eaters, Red-headed trogon, Mustached barbet, Orange-headed thrush, and the Great horn-bill.
If not for the animals, then you must visit the National Park for the waterfalls and the magnificent landscape and photo opportunities that they provide.
Apart from the waterfalls and the flora, you can also indulge in some extensive trekking and hiking around the national park, visit the Bat caves (near the northern entrance of Khao Yai), take a night safari, and may be make it to the footprints of a Siamopodus khaoyaiensis (it’s a DINOSAUR) near Wang Haew falls.
The nearest town to the national park is Pak Chong which is the northern entrance to the park. You can take a train from Bangkok to Korat which goes via Pak Chong. From the station, you can hire a cab or bike or songthaew to the park. You can also take a bus from Bangkok Mo Chit bus station to Pak Chong which is more expensive but faster. Minibuses leave from Victoria Monument in Bangkok.