【Objective】 To analyze the serological characteristics and molecular mechanism of a novel B subtype allele 803delC. 【Methods】 ABO blood group was detected by serological method. Sequence-specific primer polymerase chain reaction (PCR-SSP) was used to detect ABO blood group genes. The coding region of exon 1-7 of ABO gene was detected by Sanger sequencing to determine the mutation site. 【Results】 Serological identification of patients was with forward O-type and reverse B-type. The result of PCR-SSP genotyping was A/O. There was A gene, which was not consistent with serological results. Further Sanger double-strand sequencing revealed that the C-base was deleted at position 803 of exon 7 on the basis of ABO^*B. 01/ABO^*O. 01.01. The mutation eventually leads to the amino acid substitution of p. Ala268Gly and p. Phe269Ser and the production of new open reading frame starting at position 269, with the new open reading frame No.20 amino acid being stop codon, resulted in the termination of B gene expression. Further single-strand sequencing of the ABO gene revealed that the mutation was located in the ABO^*B. 01 gene. The mutation was submitted to the NCBI database with the number OR343908. 【Conclusion】 A new ABO allele leading to B variant has been found in Chinese population. Genetic detection can be used to identify the ambiguous blood group with discrepancy between forward and reverse blood grouping.