Abstract: The intensity of low energy gamma rays in gamma gamma logging is related to the atomic number of the formation being measured.Based on chemical analysis data of 164 coal sample from 8 mines,calculation results show that the method in which equivalent atomic number is used for in situ ash content evaluation is not sensitive to the rank and moisture content of the coal but quite sensitive to the ash composition,especially to Fe₂O₃ content.Therefore,regional correlation should be established when in-situ ash content prediction is conducted.Measuring the intensities of both high-and low-energy gamma-rays,the two parameters of formation density and equivalent atomic number,which is related to the formation lithology, can be derived at the same time.