Abstract: It is obtained from a comparison of the data from base line measurement and resistivity strain gauge that first,the ratio of resistivity to strain,i.e.,the (hagni-fication) factor K=(△/) △ for small strain in some naturally unsaturated rock-soil layer is relatively high with a maximum of 103,and second,the smaller the strain is,the higher the magnification factor k will be. The relationship between resistivity p and strain has been found from an analysis of the second fact as -(E+F0.4),where is the porosity,E,F are constants. This shows that the structural coefficient ?is no longer a constant as thought previously. It is in fact a function of strain,i.e.n= E+F. Therefore,the high magnification factor is thought as a generally accepted law concerning the unsaturated rock-soil layer under small strain rather than a particular feature of rocks. It is pointed out that an observation location with a magnification factor of no less than 10 could be chosen for resistivity measurements so that an anomaly of 5% will hopefully be observed near the epicentre before an earthquake.