Abstract: The size effect of reinforced concrete (RC) members is mainly caused by: 1) the heterogeneity and the mechanical nonlinearity of concrete; 2) the complex mutual effect between steel rebar and the surrounding concrete. The use of high-strength concrete (HSC) is getting increasingly popular as the structural dimension increases, which in turn makes the size effect behavior more and more obvious. Moreover, the majority of RC members are under complex loading conditions, which makes the size effect more complicated. Experiments on a total of 16 geometrically similar high-strength RC columns with the maximum cross-sectional size of 800 mm× 800 mm were conducted. The mechanical behavior of the high-strength RC columns subjected to axial monotonic and repeated cyclic compressive loadings were studied and the size effect on axial compressive strengths was analyzed. The results indicate that: 1) the failure patterns are closely associated with the loading pattern of the high-strength RC columns; 2) as the dimension of the high-strength RC columns increases, the ductility is getting stronger, however, the energy dissipation capacity is getting weaker; 3) the nominal compressive strengths of the high-strength RC columns obtained under repeated cyclic compression are slightly lower and have a smaller discreteness degree than the ones subjected to monotonic compressive loading, and the size effect is more obvious; 4) the test observations indicate that the bi-logarithmic plot of nominal compressive strengths for large-scale high-strength RC columns follows closely the “size effect law (SEL)” proposed by Ba?ant.