Abstract: In this paper, the problems about strain observation in a borehole with two cases have been discussed. The physidal sense of rhdial observation in the borehole is described. The authors derived the expressions for radial displasment and volume strain on the interior wall of the bore hole case due to the external stresses. We gave the calculating method of the theoretical values of strain in the borehole under the tide generating forces and the formulas for calculating the values and the directions of additional principal stresses in the observed regions, according to the variation of observation values of three elements in different directions at the horizontal plane.