Background : Recent studies suggested that platelets can be activated during the preparation of platelet concentrates(PCs). We designed this study to assess the amount of platelet activation induced by some varialbles during the preparation and storage for the purpose of quality improvement of PCs. Methods : Using 66 fresh whole blood, we analysed the effects of the variables including the delayed time before preparation of PCs, centrifugal force, pH, 1 -hour incubation at room temperature before platelet resuspension and platelet agitation stop. After preparation of PCs under various conditions, the percentage of p-selectin-positive platelets was determined by flowcytometry using monoclonal antibodies CD62. Results : The effects of delayed time within 8 hours before preparation and omission of 1-hour incubation on activation of platelets were minimal . However, the effects of centrifugation speed and time were very significant. After centrifugation of platelet-rich plasma at 5,000 g for 5 or 7 minutes, percentage of p-selectin-positive platelets was increased up to 40.6% or 72.6% on day 1, and 59.7% or 87.7% on day 5, respectively.4 good correlation was observed between pH and percentage of p-selectin expressed platelet(r=-0.8584, p=0.0001 on day 1 ; r=-0.7199, p=0.0037, respectively). We also observed that platelet agitation stop for 6 hours could activate platelets in PCs significantly. Conclusion : High centrifugal force and platelet agitation stop were important variables which could activate platelets significantly during preparation and storage of PCs.