Lum Num Nan National Park located in Muang District, Phrae Province, and Thapla District, Nampard District, Uttaradit Province, and consists of complicated mountain range covered by forest, such as hill evergreen forest, dry evergreen forest, mixed deciduous forest and deciduous forest, which is higher than the highest water-level of Sirikit Dam. The area is the source of Yom River in Phrae Province and Nan River in Uttaradit Province, and about 999.15 squares kilometers or 624,468 Rais (all includes 203 squares kilometers of Sirikit Dam). It has been declared a national park since September 30, 1998.

The area is the source of water of Nan River and Sirikit Dam, and consists of productive forest, such as mixed deciduous forest, deciduous forest and hill evergreen forest, which has economically valuable plants, and many kinds of animals.

The area is in the area of monsoon climate which is damply hot and sultrily hot, the hottest is on daytime. The rains is from May to October, approximately 1,440 mm. of rain per year. The winter is very cold on the top of the mountains and mountain range which are suitable for traveling.

The north part of Sirikit Dam has four types of forest such as dry evergreen forest, mixed deciduous forest, deciduous forest and grass forest. There are economically valuable plants such as teak wood, Makhaa Mong, Burmese ebony, rosewood, ironwood, Siamese sal, ingyin etc. And the animals found are mammal, reptile and bird, such as, deer, mountain goat, wild boar, Asiatic black bear, Malayan bear, palm civet, monkey, porcupine, fishing cat, pheasant, jungle fowl, mountain turtle, many kinds of snake and more than 200 kinds of bird.