Abstract: An analytical study on the flow similarity in ignition process is completed inrespect to the effects of the air flow on the local fuel and oxygen distribution near the plug. For lean light off limitation, a momentum criteria is derived from the motion equation of fuel droplet. The criteria states that the ignition velocity rises with the increase of altitude. It is in quite good agreement with the experimental data. Moreover, it can be explained as that the ignition process changes gradually with the decrease of pa into-optimum range when the fuel flow is kept constant. This case is a very conventional fuel control scheme for ignition ot the jet engine.