Aim To study the effects of carotid atherosclerosis on activated patelets and the relation with acute cerebral infarction. Methods Colour doppler flow imaging (CDFI) was used to assess carotid atherosclerosis and flow cytometry was employed to detect CD62p and CD63 expression on circulating platelet in 115 patients with acute cerebral infarction and 30 controls. Results The plaques of internal carotid artery (ICA) were significantly increased in patients with acute cerebral infarction than those in controls. 68.75% of left or right hemispheric infarctions was ipslateral to their plaques and 58.47% of the patients with bilateral hemispheric infarction had bilateral ICA plaques. The blood flow velocites were significantly increased in the throat of stenosis of ICA plaques. CD62p and CD63 expression on platelet were significantly higher in patients with ICA plaques than those in normal carotid controls, the positive percentages and fluorescence intensity of CD62p and CD63 were 4.72%±4.52%, 5.38%±3.73% and 1.50±0.26, 1.38±0.14 respectively in subjects with unilateral plaques and were 7.72%±4.96%, 6.78%±4.02% and 1.59±0.37, 1.41±0.18 respectively in subjects with bilateral plaques. Rates of platelet activation in the patients with brain infarction was higher than those in the controls and was related to the severity of carotid atherosclerosis. Conclusions The plaques of carotid atherosclerosis result in the complex patterns of high wall shear stress followed by low wall shear stress. Circulating platelets can be activated under the complex patterns of shear stress, which surgests shear-inducd platelet activation might play a pathogenetic role in onset of acute cerebral infarction.