Based on the bi-stable characteristic of quantum-dot cellular automata (QCA), the encoder and decoder are designed according to different principles. Simulation is performed using semiclassical simulation method. Compared with the existing designs of decoder, our design requires fewer components and simpler structure. Besides, with the idea of modularization design, III/VIII encoder can be constructed using II/IV encoder. We also prove that other kinds of encoders with more bits can be built using II/IV encoder module. Moreover, the method of analyzing and designing complex circuit using the principle of the combination of basic circuit modules is meaningful for practical work. The proposed circuits are simulated QCADesigner and positive results are obtained.