Abstract: Six reinforced concrete (RC) long columns were tested to investigate the size effects of their seismic performance under seismic loads. Cross-sectional shape and section size were considered in this study. The test results show that all RC long columns undergo flexural failure. The failure mode of square columns and circle columns is different, i.e., the location of spalling concrete is not the same, and the number of cracks on columns of different sizes is also different. When the section size is the same, the bearing capacity, ductility coefficient, energy dissipation capacity and stiffness of square columns are larger than those of circle columns, and the stiffness degrades faster after reaching the peak load. The nominal bending moment, ductility coefficient and average energy dissipation coefficient of RC columns decrease gradually with the increase of the section size. The decline trend of square columns is faster than that of circle columns. The size effect of seismic performance of RC columns is obvious. A correction formula for flexural load capacity of RC long columns is proposed in combination with Bažant's size effect law, which can increase the safety performance of RC long columns with large size.