Particle aggregation is a widespread phenomenon spontaneously occurring in nature, and it is also widely explored in industry, for instance, metallurgical engineering and colloid chemical engineering. In this work, deformation behavior of Al2O3 aggregates in liquid steel is investigated as a case study. Using high-temperature confocal laser scanning microscopy, the morphologies of Al2O3 chain aggregates during their deformation are observed in situ at liquid steel/Ar interface. Thereafter, the change of morphology of aggregates over time is quantitatively investigated, and the results show that circularity of aggregates increases during deformation. This fact indicates that the morphology of aggregates tends towards spherical with time. The bending forces between the particles in an aggregate during the deformation are calculated based on the experimental data obtained from in situ observations. The force exerted to cause bending is predicted by the capillary force model established for attractive capillary force of inclusions. It is found that the bending force is slightly lower than the capillary force, and this indicates that the capillary force plays a significant role in the bending of aggregates during deformation. Moreover, the difference between the two forces is due to a resistance force existed from inflection part of the aggregate during deformation. The residence is quantitatively evaluated, and there is a clear tendency showing that the resistance force increases with decreasing distance between two particles during bending. In addition, initial sintering between contacted particles after bending is reported, and the radius of neck formed between sintered particles is evaluated. Finally, the effects of inclusion size and steel flow velocity in actual steelmaking process on the radius of neck and shear stress exerted on the sintered neck during deformation are discussed.