Abstract: According to difference in features of illite such as spatia l distribution, crystallinity index, contents of swelling layer, 2M1 polytype a nd relationship between its index and copper grade, there exist two typical kind s of illite within Tongchang porphyry copper deposit, Dexing County. The one is a kind of altered mineral within the hydrothermal alteration zone. It is develop ed in the altered granodiorite-porphyry and altered metamorphic tuffaceous phyllite near the contact zone with the porphyry rock body. Its crystallinity va lue and expand ability are mainly effected by water/rock ratio or fluid influx. This kind is fo rmed by illitization of plagioclase and micas during the hydrothermal fluid evol ution and water-rock interaction within the porphyry body and the wall rocks n ear contact zone. The other kind is a product of low grade metamorphism by illitization of smectite. Its cr ystallinity index is lower than the former. It is 2M1 polytyped and witho u t swelling layer, developed in the altered metamorphic tuffaceous phyllite at ce rtain distance from the prophyry rock body (>2km). Moreover, because the illit ic crystallinity an d the degree of copper mineralization are both controlled by the water/rock rati os, the negative correlation between illite index value and copper grade indicat es that within the porphyry body and near the contact zone with wall rocks, the low er the illitic crystallinity value, the stronger the alteration degree, and the higher the copper grade. Consequently, the illitic crystallinity value could be used as an indica tor for mineral deposits research, such as for the porphyry copper deposits.