The effect of nozzle contraction angle on air entrainment rate by a vertical plunging liquid jet was investigated. The surface roughness of the jet increases with increasing nozzle contraction angle up to about 40 degrees, and then levels off. The volumetric air entrainment rate increases in proportion to (sin theta)(0.21), corresponding to the increased surface roughness of the jet. The air entrainment rate is correlated with empirical equations, which cover a wider range of the nozzle geometries and operational conditions. The relationship between the air entrainment rate per power consumption and nozzle contraction angle was also investigated. The power efficiency for air entrainment is found to be almost independent of the nozzle contraction angle.