Abstract: Based on the theory of elastic plates and shells,the concept of equivalent isotropic solid plate elaborated for perforated tubesheet,the new methodology to determine the effective elastic constants by modal analysis of such plates is described.The computing formula of effective elastic constants for plates with hetero-diameter dual-holes are given in terms of the stiffness superposition method.By modal approach,the effective elastic constants of support plates with various perforation shapes popularly used in steam generator,such as hetero-diameter holes,trefoil and quatrefoil holes,are obtained.The universal relationship between the effective elastic constants and the mass fraction are established,which can be used for perforated plates with any arbitrary hole shape.The study results indicate that the curves for calculating effective elastic constants adopted in ASME are improper in non-circular hole perforated plates and shall not to be applied in the support plates and tubesheet of steam generator.