The experimental results of air entrainment into a liquid jet flowing down onto a rotating roller were presented by means of the impinging jet method. The impinging jet was a substitute for a liquid curtain in an industrial coating process. Roll velocity at the onset of air entrainment was measured for different flow rates, nozzle diameters, nozzle heights, impingement angles, viscosities and surface tensions. The results indicated that the inertial force of the jet can delay the onset of air entrainment and that it acts most effectively on the dynamic wetting line when the flow rate is set to make the wetting line locate at the impingement point. The effect of inertial force decreased with further increasing flow rate and the onset velocity reached a constant value which depended only on the physical properties of the fluid. The results of dimensionless analyses indicated that there are two critical values of the Reynolds number, i.e., the one at which the onset velocity has a peak and another above which the onset velocity does not depend on the Reynolds number. In the Reynolds number above the second critical value, major forces acting bn the dynamic wetting line were viscous drag force and surface tension, and the flow mechanism at the onset of air entrainment was the same as that in plunging tape flow. Application of the results to curtain coating flow indicated that the critical Reynolds number exists not only in plunging jet flow but also in plunging curtain flow.