Abstract: After the Tangshan Earthquake of 1976, strain adjustment took place in the earthquake source region by the occurrence of a series of aftershocks. The relative strain changes in the earthquake source region may be calculated from the energies of the aftershocks by the Benioff's formula. On the whole, these changes are consistent with those obtained from the vertical deformation observed at a geodetic levelling point within the earthquake region. The correlation between the variations of the radon content in water and the changes of relative strain is rather good. Further discussion based on such observations suggests that the increase in radon content in water after the main shock may be related to the strain adjustment in the earthquake source region and the cumulative increase of macroscopic fractures of the water bearing rock formations.