This paper presents an experimental investigation on the effect of side wall length (normal to the facade direction) on the ejected facade flame height from a window of an under-ventilated compartment fire. The ejected facade flames are recorded by a CCD camera with various side wall lengths L at different side wall separation distances D. Results show that, as the length of the side wall increases, the change in behavior of the flame height can be categorized into two regimes: (1) the flame-entrainment-controlled regime in which flame height increases with side wall length for "(half) axisymmetric fire" but independent of side wall length for "wall fire"; and (2) the combustion-efficiency-controlled regime where the flame heights for both these two fire types decrease with side wall length due to decrease of the combustion efficiency inside the fire room, for L = 3D or greater. A global parameter K is then proposed based on scaling analysis for the air entrainment of the flame with side wall length, in relation to characteristic length scales l(1) and l(2) of the window, which well collapse the experimental data for different window dimensions, side wall lengths and separation distances. (C) 2014 Elsevier Ltd. All rights reserved.