The interaction between longitudinal optical phonons in an insulator and electrons tunneling through the insulator is investigated on the basis of a simple one‐dimensional model. This interaction is found to alter the mechanism of metal‐insulator tunneling drastically, the tunneling current being constant at low temperatures and highly temperature‐dependent at room temperature. The low‐field (≲106V/cm) current‐voltage relationship has an algebraic form similar to that for thermionic emission but differs from it in magnitude and temperature dependence. It is concluded that the high‐ and low‐temperature regions of charge transfer from metal to insulator are continuous, representing the evolution of the electron‐phonon interaction with temperature.