Abstract: The purpose of this paper was to investigate the stiffener impact damage and its effect on compression after impact (CAI) behavior of T-stiffened composite panels. For single T-stringer stiffened panels, low-velocity impact was conducted on the stiffener with a drop hammer over the panel at five energy levels. The impact experiments reveal that, the panel dent is almost invisible after stiffener impact. Besides, before the impact energy reaches the threshold value, there is no damage occurring in the stiffener and stiffener-panel is not debonding either. However, once the impact energy exceeds the threshold value, the stiffener will be damaged under bending tensile stress, meanwhile the stiffener-panel will debond severely. Compression tests were performed on intact and damaged specimens. The tests show that, when no damage is induced in the stiffener, the low velocity impact has little effect on buckling behavior of the stiffened panel while the failure load is reduced slightly. However, after the stiffener is damaged by impact, the stiffener will unload due to the damage growth during the compression process. The unloading of stiffener can weaken the support for the panel and thus decreases the panel buckling load, which can dramatically reduce the load carrying capacity of the stiffened panel.