Abstract: Objective To investigate the serological characteristics of one case of B（A） blood type identified by DNA sequencing. Methods The ABO blood phenotypes were determined by routine serological methods，and the ABO genotype was determined by direct sequencing and cloning sequencing. Results The serological results of positive and reverse ABO blood typing were inconsistent．The positive serological result presented AB，while the A antigen was relatively weak and the H antigen was stronger than that of the normal type B. The reverse serological detection showed the existence of weak anti-A1. Complete regions of sixth and seventh exons of the ABO gene were amplified .The sequencing result showed that allele O02 was found in the sixth exon，and 640 A>G mutation was showed in the seventh exon. It showed an allele of B（A）04 and genotype of B（A）04 /O02. Conclusion The blood type was identified as B（A），and further genotype was identified as a rare B（A）04 /O02.

Key words: B（A）type , ABO genotype