This paper describes the development of joint design approach for aluminum space frame. The space frame consists of thin-walled closed extrusions joined by welding. A welded joint cannot be treated by the rigid joint assumption for the effect of the weld line.The continuous fillet parts are modelled as interface elements using detailed finite element model. The study presents the equivalent effective Young's modulus and the effective zone with equivalent modulus. The structural joint stiffnesses of several pillars are calculated by the present joint design method and examined by stiffness test. The effects of fillet size, wall thickness and pillar geometry are considered.The results provide a useful and general method to predict the stiffness of joints with various configurations, and practical guidelines to analyze the stiffness of aluminum space frame.