Abstract The investigations on the Coulomb blockade (CB) effect, which strongly affects the transport properties of the quantum devices, are generally restricted to the regime where the Coulomb repulsion U is far larger than the temperature T, yet the critical status was not discussed. We represent the boundary between the CB and Non-CB regimes with the nonequilibrium Green׳s function approach based on the equation of motion by employing a quantum dot model. The boundary line can be expressed with a equation Γ / U = f ( T / U ) , and then the systems will be in CB regime when Γ / U f ( T / U ) . Thus, this boundary allows us to easily know when the CB effect is significant in the electronic device investigations.