The deformation behavior of beam-column sub-assemblages has a significant impact on the lateral displacement of frame structures. Understanding and realistically predicting the deformation constitution of beam-column sub-assemblages are important aims in the control of frame structural lateral displacement. In this study, experimental tests on three reinforced concrete frame structures were carried out to investigate the deformation constitution of beam-column sub-assemblages. The specimens are different for the axial compression ratio and beam-to-column stiffness ratio. A numerical model was developed to further study the effects of axial compression ratio, reinforcement ratio of column, and shear-to-span ratio of beam on the deformation constitution. In addition, a method to calculate the deformation constitution was proposed.