Abstract: Entrainment rate and flow regime on large hole flow-guiding sieve tray were investigated in an air/water simulator. The results showed that the flow regime on a tray was interrelated with the change of entrainment rate. At certain gas velocities, increasing and/or decreasing the liquid load gradually, it was found that the entrainment rate would change as the shape of a saddle. The clear liquid height, Zc, corresponding to the lowest point of saddle is about 30-40mm. This point is defined as the turning point of the entrainment rate. And the gas-liquid phases while clear liquid height, Zc, is less than turning point is spray (spray dominant) regime; conversely, it is froth (froth dominant) regime. The entrainment rate turning point is the transition point from spray to froth regime. It was observed that the regime transition was a gradual instead of a sudden change. The parameters that affect the entrainment rate and the characteristics of regime transition were discussed. It is proposed that, in general, the prevalent entrainment correlation, i.e. Hunts equation, may only be applied to the froth regime but not to the spray regime.