Modern hydraulic cranes which can lift and move heavy objects are much used in factories and building areas. It has many merits that it can control its length easily and move easily in the city. So, the shape optimization of hydraulic crane boom is important for the weight reduction of crane boom. Design of the hydraulic crane boom is considered to minimize its weight. In this study, two section-shapes of rectangle and trapezoid are considered. In the case of rectangle-section, flange length, web length, upper flange thickness, lower flange thickness and web thickness are design variables at each boom. In the case of trapezoid-section, upper flange length, lower flange length, height of trapezoid, upper flange thickness, lower flange thickness and web thickness are design variables at each boom. Some of the loading conditions from the load-chart are selected for optimal design condition. The selected loading conditions have higher moments and shear forces than other conditions. Three failure modes with compression, tensile and shear mode which are recommend in SAE are constraints used in the optimal design. Geometric constraints are also considered. In the final optimal state, each boom of rectangle-section and trapezoid-section has its optimal shape irrespective of working angle and the weights are so much reduced. The boom of trapezoid-section is slightly lighter than that of rectangle-section.