The Gutenberg-Richter earthquake frequency-magnitude relation is one of the most universal laws in the seismicity. Explaining B. Gutenberg formula in physics, two theories have been acccepted extensively. One is the theory caused by medium heterogeneity which was proposed by K. Mogi, the other is the theory caused by stress level which was proposed by C. H. Scholz. They individually built B. Gutenberg formula from probability calculating formula along with individual reasoning. However, actual Gutenberg-Richter formula shall be relation which reflects both medium character and stress level. So two forms which were built individually by K. Mogi and C. H. Scholz are very alike in appearance, but they are essentially unilateral. This is the reason why it is necessary to study physical mechanism of b value in this paper. Starting from real geo-medium and using fracture theory, Gutenberg-Richter formula is calculated. Our result of b value expression accords with result obtained from rock breaking experiment. So K. Mogi and C. H. Scholz theory is unified in theory.