Abstract: The interfacial fracture process of stitched/non-stitched composite L-shaped skin-to-stiffener structures under pull-off load was simulated using self-defined suture model and Cohesive element method. The effects of the location of the first suture and the flange width of stiffener on interfacial strength were analyzed. The results show that for non-stitched structure,the Ⅰ&Ⅱ mode cracks existed together near the load center line and the Ⅱ mode crack was dominant far from the load center line. For stitched structure,the load waved during crack expansion process and reached the maximum value(pulling out strength) when the first suture fractured. The strength increased 118.5% when the first suture transferred from the position of 8 mm from the load center line to the tangent of web-to-skin. The strength decreased about 18.2% when the flange width of stiffener decreased from 25 mm to 20 mm,while it had almost no change when decreased from 20 mm to 15 mm.