Abstract: The filament-process int roduces the fiber undulations and the nonorthogonality of the crossover geometry into thin-shell cylinders. This research identified the micromechanical geometry in the fiber crosserover regions of filament-wound cylinders. A new method for calculating the stiffness of filament winding composites is presented. The calculating process is roughly divided into three steps. First, the 3D stiffness for the undulating layer was converted to a 2D effective stiffness through the two angles, namely the angle of inclination, the out-of-plane angle due to the fiber undulation, and the filament winding angle , the in-plane angle. Next, lamination theory was used to obtain the local laminate stiffness which was functions of the undlation direction, and the stiffness constants were numerically integrated and averaged along the length of the undulation. Third, the whole stiffness of filament winding composites was calculated through the transformed matrix between the principal coordinate axis and the direction of filament-winding. Some important changes exhibited by the stiffness matrices, especially the stiffness coupling effects due to the fiber undulation are obvious.