This paper describes an improved pseudo-static method for the seismic design of earth dams. Precise selection of the horizontal acceleration coefficient is the primary step in the evaluation of seismic behaviour of earth dams using the pseudo-static method. Previous researchers and design manuals often suggest constant values for selecting the seismic coefficient, regardless of site seismicity and the dynamic characteristics of the structures. In order to improve the available methods, a new equation is presented to determine the pseudo-static coefficient of earth dams. In this equation, variations in acceleration with the height of the dam are considered to be linear, and the gradient of this line is related to the geometrical characteristics of the dam body, soil stiffness and damping values. The input acceleration amplitude is determined, based on the seismic conditions of the region and site effects. In order to evaluate the coefficients of this equation, pseudo-static, static and dynamic analyses of the Masjed Soleiman earth dam are performed. Finally, some recommendations to good dam design in seismic areas are presented.