Abstract: The calibration technique of detection efficiency is one of the most important techniques in tomographic gamma scanning (TGS) method. The calibration of detection efficiency with Monte-Carlo method was proposed. For a 3×3×3 TGS model, all 972 efficiency matrix elements were calculated by Monte-Carlo code. The experimental calibration of detection efficiency in TGS prototype was conducted. The calculated values fit the experimental data very well with relative deviation less than 5%. The results show that Monte-Carlo method used to obtain TGS efficiency is feasible and reliable.