This study proposes a systematic sizing methodology for optimizing cross-section of ductile moment-resisting frames subjected to stochastic seismic excitation. The procedure consists of two major steps. In step 1, to minimize the material use, the moment of inertia (MOI) of each frame member is optimized with design criteria fulfilled. In step 2, based on the optimized MOI, cross-sections are determined from standard shape database to minimize material use of each column with sufficient MOI. Compared with selecting cross-sections directly from database, the proposed procedure converts the time consuming discrete optimization to relatively uncomplicated continuum optimization to be applicable. An example is conducted to illustrate the optimization procedure and demonstrate its efficacy. Optimized W-shape cross-section of frame columns are designed based on the American institute of steel construction (AISC) shape database. The optimization results show that the total material use is reduced with structural responses improved.