Covering extents of Tehran and Mazandaran Provinces, this 398853 ha region was designated protected area in 1967 thanks to its high biodiversity, various biomes and eye-catching landscapes. The region constitutes two distinct northern and southern slopes. The altitude range of -10-4300 m and mean annual temperature and precipitation of 8-17 C and 350-1100 mm, respectively, have resulted in temperate per-humid, cold humid, temperate semi-arid and warm Mediterranean climate in the region. The humid northern slope is predominantly covered with dense Hyrcanian forests. 1100 plant and 264 animal species have been identified in the region. The main plant species include beech, hornbeam, chestnut- leave oak, scotch elm, lime tree, velvet maple, coliseum maple, ironwood, ash tree, box tree, juniper, common birch, walnut, willow, yew, astragale, wormwood, Penucedanum petiolare, Centaurea elbursensis, Tanacetum archibaldii, succor, prickly thrift, borage and different species of Gramineae. The 1592 ha cypress forestes located near the city of chaloos are the sole relict and reserve of this coniferous species. Some of the main animal species of the rigion are red deer, roe deer brown bear, wild goat, Alborz red sheep, leopard, jungle cat, Eurasian lynx, common fox, wolf, wild boar, jackal, pheasant, Caspian snowcock, chukar partridge, different species of Passeriformes, warblers, birds of prey, transcaucasian meadow viper, Caucasian pit viper, steppe ribbon snake, Kukri snake, southern Caspian kutkum, carp, trout, lenkoran and eel. The presence of invaluable Hycanian forests, high flow rivers, Amirkabir Dam Lake, snow-clad mountains , proper access roads, historical monuments and outstanding natural sceneries have encouraged scientific, research and tourist activities in the region.