For compartment fires with an opening, two distinct states are widely recognized: well-ventilated (oxygen is sufficient for combustion inside the compartment) and under-ventilated (oxygen is completely used inside the compartment). And as a consequence, the flame ejects out through the opening with excess fuel burning outside when reaching under-ventilated condition. However, in this work, a further transitional behavior of under-ventilated compartment fires with increasing fuel supply is revealed and quantified. Experiments were carried out using an under-ventilated fire compartment (0.4m cubic) with a fixed opening width (0.25 m) and various opening heights (0.0125 m to 0.15 m) (corresponding ventilation factors, A root H, 3.49 x 10(-4) to 1.45 x 10(-2) m(2.5)). The temperature inside the compartment and the facade flame ejected through the opening were recorded with increasing fuel supply (or namely total heat release rate) for under-ventilated conditions. The results showed that when the under-ventilated compartment fire reached a transitional state, the temperature inside the compartment experienced a sudden drop associated with a sudden increase of facade flame height outside the opening, posing a more severe impact on the building facade. The critical fuel supply rate for reaching the transitional state could be divided into two different mechanisms by a critical value of the opening ventilation factor ([0.5A root H]=0.0033 kg/s), based on the mass balance and flame stoichiometric extinction analysis inside the compartment. The formula for describing the critical fuel supply rate by considering the evolution of the critical air-fuel equivalence ratio (chi) for both mechanisms was proposed. The classic model on the facade flame height for common under-ventilated compartment fires was found to be not applicable when the transitional state of under-ventilated condition had been reached. A new model was proposed to correlate the facade flame height for the transitional state of under-ventilated condition, which was shown to have a 2/3 power dependence on the total heat release rate taking the opening width as the characteristic length. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.