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The mechanism of inclusion aggregation in liquid steel in swirling flow tundish is analyzed by applying the theory of flocculation which was developed in the field of colloid engineering. The gas bridge forces due to the micro bubbles on hydrophobic inclusion surfaces were considered responsible for the inclusion collision and agglomeration, which can avoid the aggregation to breakup. The quantity of micro bubbles on hydrophobic inclusion particle is more than that on hydrophilic one. The trend of forming gas bridges between micro bubbles on particles is strong in the course of collision. The liquid film on hydrophobic particles is easy to break during collision process. Hydrophobic particles are liable to aggregate in collision. According to the analysis of forces on a non-metallic inclusion particle in swirling chamber, the chance of inclusion collision and aggregation can be improved by the centripetal force. Hydrophobic particles in water are liable to aggregate in collision. Hydrophilic particles in water are dispersed although collision happens. The wettabiliy can be changed by changing solid-liquid interface tension. The non-metallic inclusion removal in swirling flow tundish is studied. It shows that, under certain turbulent conditions, the particle concentration and the wettability between particles and liquid steel are the main factors to induce collision and aggregation.