Abstract: A finite element model simulating the behavior of un-stiffened K-joints with circular chord and square braces was established. Numerical simulation reveals failure process and propagation of plasticity of joints and failure modes. The effect of diameter ratio of branch to chord β, ratio of chord radius to thickness γ, and thickness ratio of branch to chord τ on ultimate capacity of the joints was also studied. Parametric analysis indicates that γ and τ have large effect on ultimate capacity of joints compared with β. Joints with small γ and small τ yield completely. Based on the design equation of circular hollow section K-joints by International Institute of Welding, a formula predicting the ultimate capacity of unstiffened K-joints with circular chord and square braces was proposed by applying multivariate regression analysis. Results calculated using the proposed design equations agree well with experimental results and finite element analysis results, so the design equation of unstiffened K-joints with circular chord and square braces was validated.