A calculation on the rates and the branching ratios of two-body decay of proton and neutron is presented in detail in this paper. A relativistic formalism based on the B.S. wave functions is employed to consider the bound state effects of the quarks inside the nucleon automatically and completely. In order to solve the well-known Van Royen-Weisskopf's paradox occurring in nonrelativistic quark models, a B.S. wave function of mesons with a spin structure different from the Bargmann-Wigner's is accepted and as a result changes in the. branching ratios and the rates are made, e.g. the mode becomes the largest among the two-body modes for the proton decay and the rate for the neutron decay decreases, with the effective interaction induced from the minimum SU(5) model.