BACKGROUND Our previous study showed that ultraviolet C (UVC) from xenon (Xe) flash without any photoreactive compounds inactivated bacteria in platelet concentrates (PCs) with less damage to platelets (PLTs) as compared with Xe flash containing ultraviolet A, ultraviolet B, and visible light. Here, we report a UVC irradiation system for PCs under flow conditions consisting of a flow path–irradiation sheet, a peristaltic pump, and a collection bag. STUDY DESIGN AND METHODS Platelet concentrates containing Ringerʼs solution (R‐PCs) inoculated with bacteria were injected into a flow path sheet using a peristaltic pump, being irradiated with UVC from Xe flash. The quality of the irradiated PCs containing platelet additive solution (PAS‐PCs) was assessed based on PC variables, PLT surface markers, and aggregation ability. RESULTS Streptococcus dysgalactiae (12 tests) and Escherichia coli (11) were all negative on bacterial culture, while Staphylococcus aureus (12) and Klebsiella pneumoniae (14) grew in one and two R‐PCs, respectively. Bacillus cereus spores were inactivated in 7 of 12 R‐PCs. PC variables became significantly different between irradiated and nonirradiated PAS‐PCs. P‐selectin, first procaspase‐activating compound (PAC‐1) binding, and phosphatidylserine increased by irradiation. Aggregability stimulated by adenosine diphosphate, collagen, or thromboxane A2 increased in the irradiated PAS‐PCs, while that by thrombin became smaller compared with nonirradiated controls. CONCLUSION This newly developed system inactivated bacteria including spores in R‐PCs. PAS‐PCs irradiated by this system retained acceptable in vitro quality and aggregability. Usage of a peristaltic pump instead of agitator during irradiation may enable this system to be directly combined with an apheresis blood cell separator.