The characteristics of bubble entrainment process owing to vertical circular jet at the water surface are discussed. The two-phase flow measurements using imaging technique for two-dimensional bubble measurements, so called Bubble Tracking Velocimetry (BTV) were conducted in the laboratory. The dependence of the Froude number and salinity on the air bubble statistics beneath the water surface are discussed both downward and rising bubbles. The clear dependence of the Froude number on bubble size distribution is obtained through the experiments.