This paper presents in-plane seismic behaviors of the rocking-critical unreinforced masonry walls. Reliable approaches to accurately assess the seismic capacity (strength, stiffness and energy dissipation capacity) of the walls are investigated on the basis of the experimental results. In addition, the practical strength and stiffness formulas are proposed by modifying the theoretical formulae. In particular, a design chart, predicting the ultimate strength of the walls, is developed using the proposed strength formula. The experimental results show that the proposed formulas and the design chart are rationally predict the seismic capacity of the walls.