The work in this paper is to clarify and quantify the effect of ambient wind on compartment fires. Under ambient wind conditions, a set of experiments were conducted in a compartment with a window and a door at the opposite walls. The effect of ambient wind on flame behavior, airflow characteristic, temperature distribution and mass loss rate (MLR) was investigated. Results show that at low wind velocities the air flows into the compartment from lower part of the door while hot gases and flame flow out from the window and upper part of the door. For the wind velocity being equal to the velocity at the transitional phase, the flame can no longer flow out of the window. At high wind velocities, the flame spilled out from the door. An optimistic opening distance is determined for a theoretical model from the literature predicting the critical velocity for opposing wind force with downward flow. Moreover to predict the MLR, the heat feedback is taken into account by involving the compartment temperature in a modified model with wind velocity and the make-up air at the door. Involving ambient wind and the make-up air at the door, a new characteristic length I is proposed using theoretical analysis, which is related to the amount of make-up air at the door. The characteristic length I can be used to estimate wind effects on fuel mass loss rate and it reflects that increasing the wind velocity would push the flame flowing out from the door and restrains the air entrainment at the door way thus reduces the MLR. (C) 2015 Elsevier B.V. All rights reserved.