The present invention relates to a method for preventing and/or treating thromboembolic diseases, comprising the following step: administering a coagulation factor XI (FXI) binding protein to a subject in need thereof, which binding protein can comprise at least one immunoglobulin single variable domain capable of specifically binding to FXI, wherein the at least one immunoglobulin single variable domain may comprise CDR1, CDR2, and CDR3 in a VHH as shown in any one of SEQ ID NOs: 4, 10, and 14.