This paper presents object-oriented programming for topology design of trusses by a genetic algorithm (GA). When a GA is used to design a truss structure, intermediate solutions during the evolution process are always evaluated against the employed design code by the finite element method. This design process by a GA requires a computer program that can perform finite element analysis and also has the necessary information of the employed design code and standard. Efficient implementation of this kind of program requires excellent data structuring. In this study, the object-oriented programming concept is used to design program structures for topology design of trusses by a GA. The proposed structures use classes to represent design codes and standards. The structures also include classes of truss design elements that are created by combining the ordinary truss finite elements with the necessary design information via class inheritance. The truss-element objects of these derived classes are therefore capable of knowing whether they satisfy the design constraints or not. This significantly improves the implementation of GAs for design of trusses.