e stress analysis method for fixed tubesheet (TS) in pressure vessel codes such as ASME VIII-1, EN13445 and GB151 are based on the classical theory of thin plate on elastic foundation. In addition, these codes all assume a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the unit or one TS need be considered. In this study, based on Ambartsumyan's theory of transversely isotropic thick plate, a refined theory of stress analysis for TS is presented which also considers unequal thickness for two TSs, different edge conditions, pressure drop and dead weight on two TSs, the anisotropic behavior of the TS in thickness direction and transverse shear deformation in TS. Analysis shows floating and U-tube heat exchangers are the two special cases of the proposed theory. Theoretical comparison shows that ASME method can be obtained from the special case of the simplified mechanical model of the proposed method. Numerical Comparison results indicate that predictions given by the refined theory agree well with finite element analysis (FEA) for both thin and thick TS heat exchangers, while ASME results are not correct or not accurate.