Abstract: Objective To investigate the changes of plasma protein C (PC) inpatients having oral warfarin after mechanical prosthetic valves operation and its relationship with the changes of other coagulation factors. Methods Plasma PC was measured by ELISA.FX:C,and PT(INR) were measured by one stage method. Results The concentration of PC of control group and oral warfarin group were(5.0±0.64)mg/L,(2.54±2.18)mg/L respectively. There was significant difference between the two groups. These concentrations of PC were(4.02±3.60)mg/L,(2.52±1.56)mg/L and (2.40±1.66)mg/L,in patients who have warfarin for 1～6 months, 7～24 months and 25～132 months respectively. The change of PC have no correlation with FX、PT and INR. Conclusion The patients having oral warfarin with mechanical prosthetic valves operation, had a lower concentration of PC, and it related with the time of having warfarin, there was great individual variation, in the change of plasma PC. Surveillance of PC may have important clinical significance in the early stage of anticoagulant treatment.