Abstract: According to EU3 (EN1993-1-8), no-diaphragm joint connecting RHS tube column and H-beam usually behaves as semi-rigid joint. This behavior is mainly caused by deformation of tube wall. In order to calculate the internal force and the displacement accurately, the elastic stiffness of the joint should be determined correctly. In the fit formula recommended by AIJ (Architecture Institute of Japan) there is deficiency which introduces the concept of strength to evaluate the elastic stiffness. Furthermore when different yield strength values of column and beam are set, the result by fit formula will produce obvious errors. Based on elastic theory, the stress distribution model is established by finite element method analysis, and trigonometric progression solution of the elastic stiffness is deduced. The progression result is in agreement with the test and numerical results.