The slug flow regime may be appeared in subsea gas-liquid pipeline of the offshore petroleum industry. The gas entrainment process and the gas bubble distribution in liquid slug are crucial for the model of slug flow. An experimental facility was constructed and the gas bubble distributions in the liquid slug were measured by the dual-tip conductivity probe. It is found that the entrained gas is broken up into small bubbles by the high turbulent shear stress in the turbulent shear layer in the mixing zone. The small bubbles are dispersed completely and the profiles of void fraction and bubble frequency have peaks in this layer. The mechanism of gas entrainment is also presented.