Abstract: Compared to the trusses with circular hollow sections, the trusses with rectangular hollow sections have both technical and economic advantages on the aspect of construction, therefore they are widely used in the bridge engineering. In terms of the demand on the fatigue assessment of this sort of structures based on fracture mechanics method, the calculation method for stress intensity factor of rectangular hollow section K-joints under basic balanced loadings is studied. The finite element model for a rectangular hollow section K-joint with surface crack is developed and validated with test results. Then, the influence of joints and weld parameters on the geometrical correction factor Y is evaluated through parametric analysis. The equation of stress intensity factor of rectangular hollow section K-joints is proposed through multiple regression analysis and, then it is modified for design purpose. Additionally, a case study is conducted to present the fatigue residual life assessment method for hollow section joints based on fracture mechanics. The research results show that the mean value of finite element results to test results is 1.012 with the variable coefficient of 0.034. The maximum difference between these two results is just 5.5%. It indicates that the finite element model is reliable. It also shows that the joint geometrical parameters 2γ and τ correlate with Y, while θ negatively correlates with Y. The reason is due to the change of joint stiffness and of tensile loading, which changes the stress field at crack tip and crack propagation rate. The weld geometrical parameter c/a correlates with Y, while a/t₀ negatively correlates with Y. It should be noted that the influence of c/a on Y is not obvious. The proposed equation of stress intensity factor can agree well with the finite element results, and the modified equation for design purpose has the reliability of 95%. The stress intensity factors of circular hollow section joints are greater than the rectangular hollow section joints with an average increase of 24.9%. It demonstrates that the crack propagation rate of circular hollow section joint is faster compared to the rectangular hollow section joint under the same loading conditions. The case study further demonstrates this conclusion that the fatigue residual life of a circular hollow section joint is 2.1×10⁵ cycles, which is lower than a rectangular hollow section joint with the fatigue residual life of 2.3×10⁵ cycles.