Abstract:  A stiffness prediction model was established for stitched foam-core sandwich composites based on the microstructure, and the equivalent stiffness was also achieved by experiments. Considering different sizes of representative volume element (RVE) and resin enriched zone caused by different stitching angles, a modified fiber waviness model was developed to evaluate the stiffness of stitching laminated face-panels, and the effect of thickness variation of the face panels after stitching was also included. The stitched foam core, stiffened by the resin cylinder, was simplified as a unidirectional composite, and its stiffness was estimated by a mixed series parallel model. Stiffness experiments of stitched foam-core sandwich composite panels were performed. The predicted equivalent moduli of the stitched face panel and the stitched foam-core sandwich panels show good agreement with the experimental data. The effects of stitching parameters and structural parameters on the stiffness were discussed.