In the world, constructions are mostly reinforced concrete framed structures with infill walls. Different reinforcement studies have been carried out on these existing structures in order to ensure the safety of life and property without heavy damage because of the earthquakes. Strengthening of the infill walls in reinforced concrete structures is also among these studies. In particular, reinforced infill walls with steel plates have been an effective, economical and convenient method. As a result of large-scale earthquakes, the infill walls of the concrete structures have been fragmented and/or separated from the concrete frames, and damage to structures has caused many people to be injured and killed. In order to avoid these problems, in the strengthening studies with the expanded steel plates, high increases in load carrying capacities, stiffness, ductility and energy depletion capacities of brick infill walls were provided and these walls were developed. In this context, in this study, it was aimed to determine the unit length change ratios of brick walls reinforced with expanded steel plates at the time of flow and rupture depending on deformations under diagonal pressure loading. In this study, the unit length changes due to diagonal deformation  under diagonal pressure loading of thirty reinforced test elements, two of which were taken as the reference, prepared according to standards of the test system developed by the ASTM E519 (ASTM 2010), were investigated. In order to measure the unit length changes due to diagonal deformations, two LVDTs with a sensitivity of 0.01 mm, which can measure 50 mm on the front and back of the wall, were connected. As the result, the unit length changes due to deformations occurring on the front face of the test elements, at the moment of flow and shortly afterwards (plastic deformations) were determined. It has also been observed that reinforced test elements retain their integrity under diagonal pressure loading, although they exhibit unit length variation due to high deformations. In this way, the reinforced infill walls will be prevented from being dismantled and the loss of life caused by the earthquakes will be prevented.