Abstract: s: The effects of four kinds of stitch parameters on the in-plane stiffness and strength of stitched composite laminates are investigated by both numerical calculation and experiments of 158 specimens in 37 groups. Results show that, (1) the greater the stitch density is, the greater the damage by stitch and the effect on in-plane stiffness and strength are. The maximum in-plane stiffness differences at several kinds of ordinary stitch density are 6%, 11% and 9% in longitudinal, transversal and shear directions, respectively. (2) The stitch density and perfection have great influences on the in-plane tension and shear strength. The maximum reductions of the in-plane tension and shear strengths resulting from stitching are 14% and 17%, respectively, while less than 4% reduction is gained for compression strength. (3) When the stitching threads go in the direction perpendicular to that of property measurement, higher in-plane stiffness and lower in-plane strength are achieved. Otherwise, when the stitching threads go in the direction parallel to that of property measurement, lower in-plane stiffness and higher in-plane strength result. The maximum differences of the in-plane stiffness and in-plane strength are 21% and 18%, respectively.