Experiments were performed on transient heat transfer caused by large stepwise heat inputs to a flat plate heater. The plate is 6 mm-wide and located in the middle of the inner bottom wall of a horizontal rectangular duct which has 3 mm gap and 100 mm length. When the stepwise heat input is larger than that corresponding to the steady-state CHF (Critical Heat Flux), the quasi-steady state exists for a certain lifetime on the extrapolation of steady-state Kapitza conductance curve. Then the quasi-steady state rapidly changes to film boiling. The lifetime was systematically measured. The measurements were made for the flow velocities ranging from 0 m/s to 1.5 m/s and for the inlet liquid temperatures from 1.8 K to 2.0 K at atmosphere pressure. The results display the relationship between stepwise heat flux and lifetime. Besides, numerical analysis was performed on two-dimensional heat and fluid flow in the duct under the same conditions as the experimental ones by using the computer code based on the two fluid model and the theory of mutual friction. It is confirmed that the calculated lifetimes agree well with those measured by experiments. Details of heat transfer mechanisms were clarified by the analysis. © 2006 American Institute of Physics.