Objective: To study the relationship between platelet activation and the degree of bleeding in patients with primary immune thrombocytopenia (ITP) . Methods: 43 patients with ITP were assessed based on ITP-BAT bleeding grading system. Platelet membrane glycoproteins (GP) Ⅰb, GPⅡb/Ⅲa and P-selectin expression were detected by flow cytometry analysis with and without adenosine diphosphate (ADP) stimulation. Association of platelet activation with platelet count, immature platelet fraction (IPF) , bleeding severity were evaluated. Results: GPⅡb/Ⅲa and P-selection expressions on unstimulated platelet in ITP patients were higher than those in healthy controls (65.69±10.73 vs 7.16±0.99, t=4.130, P<0.001; 15.43±1.41 vs 12.55±1.03, t=2.070, P=0.043, respectively) , and GPⅠb expression was lower than that in healthy controls (240.11±24.93 vs 295.11±22.15, t=2.417, P=0.020) . Comparatively to healthy individuals, following ADP stimulation, GPⅡb/Ⅲa expression in ITP patients increased (133.96±12.17 vs 39.67±4.99, t=5.256, P<0.001) , whereas GPⅠb and P-selection expressions decreased (37.09±3.94 vs 109.77±23.66, t=3.901, P<0.001; 149.06±19.14 vs 205.73±21.00, t=2.070, P=0.043, respectively) . ADP-stimulated GPⅠb, ADP-stimulated and unstimulated P-selection, proportion of GPⅠb, P-selection levels with/without ADP stimulation were significantly associated with platelet counts (P<0.05) . ADP-stimulated P-selection and proportion of P-selection levels with/without ADP stimulation were significantly associated with IPF (P<0.05) . There were significant differences in the expressions of unstimulated P-selection, ADP-stimulated P-selection, ADP-stimulated GPⅠb stratified by bleeding grades (P<0.05) . The ratios of P-selection, GPⅡb/Ⅲa and GPⅠb with/without ADP stimulation in ITP patients were significantly different among various bleeding grades (P<0.05) . Higher proportion of GPⅠb with/without ADP stimulation was associated with higher risk of bleeding (OR=3.05, P=0.011) . Lower proportion of GPⅡb/Ⅲa and P-selection with/without ADP was associated with higher risk of bleeding (OR=0.32, P=0.023; OR=0.04, P=0.006, respectively) . Conclusion Platelet activation index could accurately assess the degree of bleeding in patients with ITP, and also be used as the observation index and reference index for treatment.