Abstract: The response spectra of NaI and BGO detectors under the condition of different energy γ-rays were simulated by using Monte-Carlo method. It is concluded that the counting efficiency of photoelectric and the first escape peaks for BGO crystal is relatively higher, and that of the first and the second escape peaks for NaI crystal makes great contribution. Under the condition of the borehole and sandstone formation filling with oil and water, respectively, the ability is different when different energy windows are selected to distinguish formation by simulating the detector response spectrum of various diameters and length of NaI and BGO crystal. The C/O value difference of BGO crystal is higher than that of NaI crystal when the energy window includes photoelectric peak, the first and the second escape peaks, but it is less affected by the different sizes of detector crystal. Using photoelectric peak corresponding to the energy window, the C/O value difference of BGO crystal detector is much larger than that of NaI, but it increases with the crystal size increasing. The C/O value is greatly affected by energy drifting, while relatively a little by energy resolution.