Abstract:The very low to low- grade metamorphism overprinting on the Precambrian in Fanjingshan area was investigated with illite crystallinity, chlorite crystallinity, b0 value of K- micas (illite) and geothermometry of chlorite. The results demonstrate that illite crystallinity Kübler index (KI), calibrated with Kisch IC set, ranges from 0.20~0.99°Δ2θ, and the rkai index of chlorite crystallinity ranges as 0.22~0.63°Δ2θ, indicating the Precambrian in Fanjingshan area encountered with the geological processes from diagensis to epimetamorphism, and it remains diagenetic, anchizonal and epizonal records in rocks in the area studied. The diagenetic rock contains diagenetic index mineral illite- montmoriolite mixed- layer phase while anchizonal rock bears metamorphic index mineral paragonite. The b0 of K- micas (illite) ranges 0.9000~0.9045 nm, and on average 0.9018 nm or roughly equivalent to 366 MPa. The cumulative frequency curve of b0 stands in between the curves of N. New Hampshire and Otago, indicating the metamorphic pressures condition in between low- intermediate pressure and intermediate pressure facies. Peak metamorphic temperature was evaluated at 293~364℃ and average 342℃. Therefore the palaeo- gradient was derived at 27℃/km. The anchizonal boundaries across strata and folding axis of the Fanjingshan anticline, clarifying the syn- to post- sedimentary or structural movement of epimetamorphism, which caused the increasing in depth and the rising of temperature and pressure in rocks at the bottom or lower part of basin due to structural burial.