Resonant tunneling in rectangular multi-barrier structures are studied theoretically. Analytical expressions of the transmission coefficient and the resonance condition are strictly derived. It is found that the resonance energies are different from the eigenvalues in the quantum well due to coupling between wells in the multi-barrier structures. The resonance condition corresponds to the energy range of the physically possible wave functions in the Kronig-Penney model. It is confirmed that the transmission spectrum is a Lorentzian nearby energies of resonance. We believe that the expression are useful for evaluating the energy variations of transmission coefficient and estimating the resonance energy, and also useful in fabrication of resonant tunneling devices.