The purpose of this study was to find the rare individual JK(a-b-) phenotype of proband family and explore its molecular mechanism and the genetic background, in order to provide base for searching compatible donor to blood transfusion of the individuals with rare JK(a-b-) phenotype. Urea lysis test was used to screen the JK(a-b-) phenotype and results were confirmed with serological method. The genotypes were detected with PCR-SSP. The 4-11 exons and their flanking intron regions of JK gene were amplified and sequenced. The results showed that her elder brother has a same phenotype JK(a-b-) and genotypes JK(a)/JK(b) with proband. The phenotype and genotypes of their parent is JK (a+b-) and JK(a)/JK(b), respectively; and the younger sister's is JK (a+b-) and JK(a)/JK(a). Acceptor site of intron 5 3' g > a mutation was detected in proband and her elder brother, which may cause the JK(a-b-) phenotype of proband and her elder brother. There is g/a and a at this site in their parent and younger sister, respectively. Additionally, the SNP (ncbi:rs8090908) a > g at nt-99 in intron 3 was found in proband and her elder brother, it needs to be explored whether the SNP is related to JK(a-b-) phenotype. This SNP was not found in their parent and younger sister. This JK(a-b-) phenotype abides by the rule of dominant inheritance in the family, suggesting that there is higher probability to find homology phenotype and genotype by investigating in their family, especially in their siblings.