The design composition elements and the mechanical behavior of trusses have been investigated for Pratt trusses. As a result, it was determined that the design composition elements of the trusses consist of the composition of webs, the distance between joints, the chords profile, the depth of the truss, and the double chord composition. In addition, by analyzing models with a variation of elements, comprehensive features of structural behavior have been presented for variations of design of Pratt trusses. This is to provide more effective and useful design information on truss structure in the architectural and structural planning stage.