The permeability of Westerly granite was measured under cyclic loading conditions for confining pressures Pc to 96 MPa and pore pressures Pp to 86 MPa in order to determine the coefficient α of the effective pressure equation, Pe =Pc -αPp, where Pe, Pc, and Pp are effective, confining, and pore pressures, respectively. Two methods of determining α are described, based on the cubic root law, k1/3 =A+B ln(Pe), which relates permeability k to effective pressure in fractures. A and B of this equation are functions of the geometric parameters of the cracks; α was close to 1 under all conditions of pressure and loading history. Pressure cycling had significant effects on the permeability of the granite, as demonstrated by the large variations in the coefficients A and B of the cubic root law.