The present study experimentally investigated the flame downwash transition and its maximum length that can be reached at leeward side of nozzle with increasing fuel supply of non-premixed jet in cross flows. Experiments were conducted with 8, 10, 13 and 15 mm nozzles, using propane and methane as fuel. The cross flow speed ranges from 0.70 to 3.08 m/s while the fuel jet velocity ranges between 0.04 and 4.26 m/s. Results showed that, with increasing fuel supply at a given cross flow speed, flame downwash length first increased (fuel supply controlled regime) then decreased (leeward side reversing flow rotating strength controlled regime). A function between the jet-to-crossflow momentum flux ratio (R-c) and dimensionless fuel mass flow rate ((m) over dot(c)*) was proposed to illuminate the tuning state of above transition. The flame downwash maximum length at the turning state normalized by nozzle diameter was found to have a linear relation with Froude number of fuel jet (Fr-j = u(j)/root gD) or cross flow (Fr-a = u(a)(2)/gD). These findings contribute to a better understanding of flame downwash evolution process, which could have a basic significance for reducing fuel emission, improving combustion energy efficiency and saving energy by controlling the flame downwash scale. (C) 2018 Elsevier Ltd. All rights reserved.