Abstract: An unique physical parameter for describing the stress level of an earthquake region is deduced on the basis of macroscopic shear fracture conditions of a fault in accordance with the theory that earthquakes are originated by fault displacements. The physical process of seismic activity ha been studied in detail. Due to the lack of the necessary original data, difficulties have been encountered in the study of the earthquake sequences deterministically. For the present state of understanding, earthquake prediction by b-value may be considered as promising in evaluating earthquake hazard.Several physical models are suggested. The physical background of the variation, of the b-value has been analysed in detail. A relation of b-value with the tectonic conditions and the stress level of an earthquake region has been deduced. Based on such result, one may be able to explain certain empirical relation and the range of variation of the b-value observed in practice more satisfactorily. If further observations-should support such result, it could provide valuable informations for earthquake-prediction.