: Although domain IV of annexin A5 (anxA5) may be less effective in binding phosphatidylserine (PS)， the four domains together may guarantee the maximum binding of anxA5 to the PS membrane. Additionally， previous research has shown that annexin mutants lacking one or more domain(s) have different biological activities compared to the wild-type. The present research mainly aims to study the role of domain IV in the crucial PS-binding function of anxA5. : The domain IV-truncated anxA5 protein was constructed and purified. Isothermal titration calorimetry， flow cytometry and activated partial thromboplastin time were adopted to examine the function of domain IV in anxA5-PS binding directly or indirectly. : The domain IV-truncated form of anxA5 is impaired in binding PS liposome and apoptotic cells， and anticoagulation activity. The mutant cannot bind calcium， but binds PS only in the presence of calcium. : Truncation of domain IV of anxA5 destroys its calcium-binding ability and impairs its PS-binding activity. Truncation of domain IV may induce conformation change of anxA5 or reduce the hydrophobic interactions between protein and membrane， which may explain the decrease of PS-binding affinity of the mutant.