Abstract—Concrete filled steel tube (CFST) columns have been gradually adopted in modern civil engineering structures mainly in high-rise buildings and bridges. Past research have shown that circular CFST columns were far more superior to its square or rectangular counterparts. Besides lower confinement effect, square and rectangular steel tubes with large width-to-thickness ratio were prone to suffer from local buckling. In order to improve the ultimate load carrying capacity of the these columns, the CFST columns were reinforced by longitudinal steel plate embedded into the concrete core. The mechanical properties such as ultimate strength, stiffness and ductility of the square CFST columns with and without longitudinal plate reinforcements were compared and studied.