Objective:To study the correlation between platelet activation status and ulcerative colitis(UC). Methods:P-selectin(CD62p) was determined with flow cytometry,and Platelet Count was tested by automatic blood cell counter including 54 UC patients in active stage,26 UC patients in remittent stage and 30 healthy controls. Results:The Platelet Count of the active UC group was (212.2 ± 97.6) × l09/L,significantly higher than those of the remittent UC group and the control group(168.7 ± 67.8) × l09/L and(166.8 ± 81.4) × l09/L,both P < 0.01). However,there was no significant difference between the remittent UC group and the control group(P > 0.05). The P-selectin(CD62p) of the active UC and the remittent UC groups were(13.3±3.4)% and (8.6±2.1)%. Both significantly higher than that of the control group [(3.9 ± 1.2)%; both P < 0.01] and there also was a significant difference between the active and remittent groups(P < 0.01). In the active UC group,the more severe the state of illness,the higher levels of the Platelet Count and the P-selectin. Conclusion:Increased platelet activation is involved in active ulcerative colitis. P-selectin(CD62p) and Platelet Count may be of great value in judging the activity state and the severity of UC.