Abstract: The stress relationship equation of reservoir rocks is that the total stress of reservoir rocks is a porosity?weighted average of skeleton stress and pore pressure. The stress relationship equation of reservoir rocks can be used in any state of reservoirs, not only in primary state. But such an equation is not directly used by researchers, while the effective stress equations are commonly used due to impossible measurement of skeleton stress of rocks. The well known effective stress equations are Terzaghi equation and Biot equation. The theoretical equation of rock compressibility derived by the stress relationship equation of reservoir rocks shows the correct logic relationship, namely, the higher the rock porosity, the larger the compressibility. Moreover, when rock porosity is zero, its compressibility is zero, too