In performance-based seismic design of slender RC walls, a nonlinear time history analysis generates larger shear force demand than the shear force demand calculated by an elastic modal analysis. In order to investigate the factors that contribute to the dynamic shear amplification effect, a parametric study was performed by considering four parameters: total number of stories, fundamental period, flexural over-strength ratio, and soil condition. A nonlinear time history analysis was carried out by using 20 different ground motions. The results showed that the total number of stories and flexural over-strength ratio were the main contributors to the shear amplification of slender RC walls. Hence, an equation for evaluating the base shear amplification factor was proposed in terms of these two main parameters. A shear distribution model was also proposed by incorporating multiple modal shapes of RC walls in the elastic motion and in the damaged motion after yielding of reinforcing bars in the walls. The proposed method showed better estimation for evaluating the shear amplification factor and shear force distribution under a severe seismic excitation.