Abstract: Full-scale tubular K-joints with tube-gusset plate connections are constructed and tested under static loads. The failure modes and failure mechanism are analyzed. Based on tubular energy theory and virtual work principle, using ring-generator separation model, this paper proposes the calculation method of the ultimate strength of the tube-gusset K-joint under three-axial load. It reveals that the ultimate strength of K-joints depends on tubular thickness, diameter, length of gusset, and main tubular stress. Moreover, for tall transmission tower, the concept of critical stress coefficient of K-joints under complex load is also proposed.