Abstract: Partial sequences of the mitochondrial DNA cytochrome c oxidase subunit I gene (COI) of seven Tegillarca granosa populations, which were collected from China's coastal areas, were amplified by polymerase chain reaction (PCR). The length of COI gene of 38 Tegillarca granosa individuals from seven populations was all 660bp. One hundred and three variable sites were detected in the nucleotide sequences of 660 bp, and 17 different haplotypes were identified. The result showed that the seven populations could be divided into two groups based on the the genetic distance and phylogenetic analysis of their COI gene sequences. The two groups were classified as Group in the Northern Fujian (including Fujian) and Group in the Southern Fujian. Group in the Northern Fujian was composed of five populations and the genetic distance was 0.0016. Group in the Southern Fujian was composed of two populations and the genetic distance was 0.0006. However, the genetic distance between the two groups was significantly high (0.1529), which suggested significant genetic differentiation between the two groups. It suggested that Group in the North of Fujian (including Fujian) and Group in the South of Fujian should be the same species, but they were different subspecies.