Abstract: In current measurement of ground stress by the borehole deformation method using pre-pressed probes of contact type, the calibration of probes have been made by Hast's method and by mid-confining pressure method. Both methods have some defects in the derivation of reduced displacements. In 1984, Pan clarified it and gave the accurate formulas (See Ref.[1]). However. the formulas can not be used in practical measurements unless the parameters involved in them are numerically expressed. This paper has worked out the numerical values of the parameters through calculation with Finite Element Method. The error of traditional formulas have been estimated. By analyzing the numerical results, the majorfactors which effect the accuracy of reduced displacements as defined in Ref.[1] have beenrevealed.