Abstract: In order to research the effects of debond size on buckling and post-buckling behaviors of composite stiffened panel, composite stiffened panel containing different debond defects were experimentally investigated. The results show that load carrying capacity of the specimens with 30 mm or 50 mm defect is almost equal to the intact panel, but the presence of 80 mm defect leads to an obvious reduction in the load carrying capacity at the post-buckling stage. By utilizing the ultrasonic scans, the damage propagation behavior was monitored. The results show that when the compression load reaches 85.3% of the failure load, signs of the expansion occur at the diagonal position located in the prefabricated defect. Shadow moiré interferometry method was applied, which shows the formation process of the buckling modes of composite stiffened panel with two half-waves and three half-waves. The monitoring results of buckling modes also reveal that the buckling mode of the stiffened panels transforms frow two half-waves to three half-waves as the debond size increases.Based on the experimental results, a finite element (FE) model was established by ABAQUS to study the numerical simulation of buckling and post-buckling behaviors of composite stiffened panels. The buckling analysis was employed to obtain the buckling load and deflection mode, and the buckling waveform was then defined on the structure as geometric imperfection to process post-buckling calculation. The numerical results agree well with the experimental results, which show the effectiveness of the FE model in the prediction of the buckling and post-buckling performances of the composite stiffened panels.