To study a family with Bw subtype of ABO blood group system, and to review safety issues in relation with clinical transfusion.The molecular basis for the blood type was studied with serological assay, polymerase chain reaction-sequence specific primer (PCR-SSP) and DNA sequencing, TA clone and haplotype analysis in one blood donor whose ABO blood group were difficulty typed and her family. The bioinformatics analysis was carried out by biological analysis software to investigate the change of structure and function of enzymes influenced by the change amino acid. A retrospective survey was carried out to investigate what is the actual position that the donor blood was used in the clinical transfusion.Three members from the family were found to have a Bw subtype. A substitution of nucleotide C by T at position 721 in exon 7 was discovered, which resulted in replacement of amino acid Arg to Trp. Review of clinical record suggested that there has been no significant abnormality association with past three blood transfusions.A 721C>T mutation of the ABO gene probably underlies the Bw subtype. Further research is needed for understanding the clinical significance of this subtype in the blood transfusion.