The behavior and ultimate capacity of single K-joint in the roof truss with welded box section members used in the National Stadium were experimentally researched. Two specimens at one-third scale of their prototypes were tested under monotonic loadings, along with finite element analysis. The test results indicated that the joint was stronger than the connected members, and the ultimate capacity of the joint was about twice as large as the design load. Neither local buckling nor fracture of the plates in the joint was observed in the tests. The results of finite element analysis match the test well. It is proved that the reinforcement of the joint such as increase of plate thickness in the joint zone and arrangement of internal stiffeners in the joint are quite effective.