Objective To investigate the molecular basis of ABO gene in a patient with serologic ABO blood group discrepancy.Methods Serologic blood group identification,Coombs' test and antibody screening were detected with DG Gel Confirm cards,Neutral cards,Coombs cards by WADiana/8XT Compact Analyzer (from Diagnostic Grifols,S.A).The enhancer,promoter,exon 1 ～ 7 and their adjacent intron region of ABO gene were amplified by using polymerase chain reaction (PCR) method.Results The patient's red blood cells was determined as weak B phenotype showing two groups in gel and mixed field in tube with monoclonal anti-B,and A phenotype with monoclonal anti-A.DNA sequencing showed nine variants in ABO gene.One heterozygous variation in exon 6 (297A＞G) and eight heterozygous variations in exon 7 (467C＞T,526C ＞G,657C＞T,703G＞A,796C＞A,803G＞C,829G＞T 930G＞A) were identified and 829G＞T was the novel variant.Compared with Blood Group Antigen Gene Mutation Database,genotype of the patient was weak expression of A102/B101.Conclusion The novel variation of B allele is the main reason of Bweak phonetype in A102/B101 genotype.Serological and molecular biological detection help to understand the characteristics of blood group phenotype and genotype,provide the guidance for clinical transfusion strategies.