A general steam generator model is developed to account for various modes such as natural circulation and pot-boiling modes which can be occurred during transients. The present model can describe not only the swell and shrink phenomena, occurred by any small change in steam flowrates and feedwater flowrates but also natural circulation, pot-boiling, and tube uncovery modes which occur in sequence during the loss of feedwater transient if auxiliary feedwater is not followed by. The void fraction concept is used instead of the quality concept to simulate counter-current flow which takes place in the pot-boiling mode after the natural circulation mode stops. The present model is based on a one-dimensional three-region model to realistically describe the swell and shrink phenomena; downcomer, tube bundle, and steam dome regions. Both of the downcomer water level and two-phase level can be predicted during the pot-boiling mode. To verify the present model in the pot-boiling mode, a simple experiment is done and simulated by the present code. It is shown that the simulated results are relatively in good agreement with the experimental data.