Abstract: The specifications of the ZX-79 string-frequency borehole strain meter are given in this paper. Its sensitivity, daily stability and coincidence accuracy are all as high as 10-9. The data observed continuously during the past two years show that the daily variations of the tidal strain due to strong and weak tide are 4-510-8 and 2-310-8 respectively. The annual variation of the strain at the surface of the earth is of order of 10-6, and the anomalous variation of the strain in the crust before an earthquake is above 10-7 order. Therefore, it would offer us a possible way to employ the sensitive borehole strain meters developed in our country for the measurement of physical parameters in earth science and earthquake prediction.