Abstract: In the current steel design code, the ultimate strength calculation methods of K-type steel tube-plate one-piece joints or detached joints with small angles (less than 30 degrees) are not involved. In this study, the load-deflection relations and failure modes of K-type steel tube-plate one-piece joints and detached joints with small angles of 25 degrees were studied. The relations among the strength, the strain of the main steel tube and the dimensions of two gusset plates were analyzed. The results show that the main steel tube was teared and damaged in shear when the dimensions of the two gusset plates were small, while large deformation and compressive buckling of the main tube occurred when the dimensions of the two gusset plates were large. The separation of the gusset plate resulted in a sudden drop of the strength. The greater are the dimensions of the two gusset plates, the smaller is the strength. Therefore, the one-piece gusset plate has good stress dispersion and transmission although there is a problem that the gusset plate is too long in the conventional small-angle K-joints.