In road tunnels, fire accidents are regarded as one of the most serious dangers to the trapped occupants, as statistical evidence indicates that the majority of fatalities result from asphyxiation due to the spread of toxic gases. The present study examines the influence of fire source elevation on the backlayering length through a series of numerical simulations. Results reveal two distinct behaviors: under low longitudinal ventilation velocity, the backlayering length increases with the fire elevation, whereas in conditions approaching the critical velocity, it exhibits a slight reduction as the fire source is raised. Based on dimensionless analysis, an equation which correlates the backlayering length to the floor-to-fire distance, the fire heat release rate (HRR) and the longitudinal ventilation velocity is proposed. Comparison of the predicted data by the proposed equation with literature experimental results shows an acceptable agreement.