Abstract: There are decline and decling diseases in pure Armand pine forest in Yunnan.The happening and appearence are basically accordant with the reports and study results of that of the decline of needdle and broad-leaved trees abroad.At first declining armand pines have significant decrease of growing.Their radius and height increase can be below 1/2 or 2/3 of the nean of the normal growing.The other symptoms of decline are the needles lose green, turn yellow, or wither and fall down, the twigs wither and die, grow weakly, the crown shape becomes small and transformed, bad trunk becomes abnormal, the bark is rough and even broken and fallen, and the cones underdevelop with decreased number of seeds, etc.These are often accompanied by various diseased and insects.The important fungal diseases are Cronartium ribicola Fischer, Cytospora pini Desm., Capnodium sp.etc.The important secondary insects are:Blastophagus piniperda Linnaeus, Cryphalus lipingensis Tsai et Li, Which are the most serious pests on declined and dying armand pines.As the "forest decling" and "decline disease" have no clear mean.The authors consider "forest decline" as the whole forest has decline symptoms and "decline disease" as individual trees have symptoms of diseases and reaction, just as the other plant diseases.