In order to optimize the sizing and topology of discrete structures together and resist the combinatorial explosion of the solution space, a co-evolutionary design (CED) method based on ant colony optimization (ACO) for discrete structures is proposed. The tailored ant colony optimization for the sizing of structures (TACO-SS) and the tailored ant colony optimization for the topology of structures (TACO-TS) are implemented respectively. Theoretical analysis shows that the computation complexity of each sub-process in CED based on ACO above is polynomial and it guarantees the efficiency of this method. After the parameter settings in TACO-SS and TACO-TS are discussed, the convergence history of a sub-process is studied through an instance in detail. Finally, according to the design examples of the 10-bar and 15-bar trusses under different cases, the effectiveness of the CED based on ACO is validated and the effects of the loads and the deflection constraints on the co-evolutionary design are discussed.