To study the relative sensitivity of permeability to pore pressure P_p and confining pressure P_c for clay-rich rocks, permeability measurements were performed on samples of four clay-rich sandstones. A new method (hereafter denoted the “slide method”) was developed and used for analyzing the permeability data obtained. The effective pressure coefficients for permeability n_k were calculated. The values of n_k were found to be greater than 1.0 and insensitive to changes in pressure. These results confirmed observations previously made on clay-rich rocks. Also, the coefficients n_k obtained had different characteristics for different samples because of differences in the types of clay they contained. The effective pressure law (σ_eff=P_c–n_kP_p) determined using the slide method gave better results about k(σ_eff) than classic Terzaghi’s law (σ_eff=P_c–P_p).