The study provides an objective, rapid, and discriminative preliminary assessment method of masonry buildings for practical engineers conducting seismic evaluation of the existing buildings. The study proposes the preliminary assessment method in accordance with static analysis theory of seismic design code of buildings. The static analysis theory is that lateral strength resistance (capacity) of a masonry building can be estimated by total cross-sectional area of vertical masonry elements. Seismic inertial force (demand) of a masonry building can be estimated by total weight of the floors above the evaluating floor of the building. The proposed seismic index is the ratio of seismic capacity to demand. Six modification factors are introduced to complement the seismic index for consideration of out-of-plane collapse, aging, and deterioration problems, etc. The major parameters of the proposed preliminary assessment method, such as unit weight of floor area, and lateral strength per unit cross sectional area of a masonry structural element, are determined by statistical regression from the building data of six existing masonry buildings in Taiwan. The six modification factors refer from the design and construction code of masonry structures in Taiwan. A masonry dormitory, as an example, demonstrates the evaluation process of the proposed preliminary assessment. The preliminary evaluation result is consistent with that of the detailed evaluation.