: The objective is to study a gene mutation (Tyr503Cys) found in a Chinese consanguineous marriage family with inherited factor XI (FXI) deficiency (cross-reacting material positive, type II). The FXI activity and FXI antigen were tested with clotting assay and ELISA, respectively. The FXI gene was amplified by PCR with direct sequencing. ClustalX-2.1-win and three online bioinformatics softwares were used to study the conservatism and harm of the mutation. The proband had reduced FXI: activity at 13%; three members had decreased to about 35%, all of whom had nomal FXI: antigen. DNA sequencing analysis showed the proband carried a homozygous c.1562A>G point mutation of F11, resulting in Tyr503Cys. Tyr503 was highly conserved among the homologous species. The three bioinformatics softwares indicated that the mutation had affected the function of the protein. The Tyr503Cys mutation was responsible for the decrease of FXI: activity, which is cross-reacting material positive deficiency and the first reported in the world.