This paper presents an experimental investigation on flame lengths of medium pool fires under horizontal cross air flows. Square pool fires with dimensions of 10 cm, 15 cm, 20 cm, and 25 cm, using ethanol and heptane as typical fuels, are burned under cross air flows ranged in 0-2.5 m/s. The burning rates are measured by an electronic balance with accuracy of 0.1 g. The flame geometrical characteristics are recorded by a CCD (Charge-Coupled Device) digital camera, in which the mean flame length is quantified based on flame appearance intermittency spatial distribution. Results show that the cross air flow enhances the mass burning rate in a linear function of flow speed. Such enhancement effect is more prominent, indicated by a higher enhancement rate (beta), for heptane than that for ethanol. This fuel type effect on beta can be accounted for by a thermochemical property-Heat Release Parameter of the fuel (R, ratio of heat of combustion to heat of effect evaporation, R = Delta H-c/Delta H-fg). The normalized value of beta/R is in a linear function of reciprocal of pool length (beta/R similar to d (1)) and independent of fuel. The flame length is found to be elongated by the cross air flow due to the enhancement of the fuel burning (evaporation), as also being more remarkable for heptane than ethanol. A generalized model (Eq. (18)) is proposed to approximate the elongated flame length (l(f)), in relation to an amended dimensionless Froude number of u/root g(d) . s Delta H-c/M-fuel Delta H-fg . M-o2/rho Y-alpha(o2,infinity) as incorporating globally both the thermochemical property (represented by R parameter, Delta H-c/Delta H-fg), molecular property and reaction molar stoichiometric ratio (represented by s/M-fuel) of the fuel, as well as the ambient oxygen molar concentration factor (represented by M-o2/rho Y-alpha(o2,infinity)). (C) 2013 Elsevier Ltd. All rights reserved.