Design of the boom of an excavator is considered to minimize its weight changing both the shape and dimensions. Loading conditions based on most frequent and significant postures are selected. The notion of possible maximum stress is suggested for implementation of constraints. Some of the loading conditions which are active and have decisive roles in terms of stress values are taken as the design loads. Using these selected loading conditions, optimum results are obtained with stress and buckling constraints, and they are later tested against all other loading conditions. The finite element model of the boom has 1694 triangular flat shell elements. The design sensitivity of the stress constraints is obtained using a modified semi-analytic method. To verify their accuracy, the results are compared with those by the finite difference method. Six different sets of design variables are studied. Slightly different optimums are obtained with weight reductions ranging from 8% to 21%. General trends are deduced. Based on these results, a designer can select his design combining with his past experience and other constraints.