This paper presents the experimental results on the panel zone behavior of steel box connections. The results of tests on twelve connections subjected to monotonic and cyclic loadings are reported herein. Based on the results, the effects of sectional-area ratio of the connecting members and the buckling of panel zone on connection behavior are discussed. This paper also provides the strength deterioration behavior under cyclic loadings and the classification of the failure mode. The results obtained from the experimental investigations indicate that the major cause of the reduction in strength and shear deformation capacity (ductility) are the failure modes and the buckling of panel zone.