In this study, the genetic diversity and genetic structure of two spawning season cohorts of Ommastrephes bartramii in North Pacific Ocean were analyzed based on mtDNA COⅠand Cytb gene sequence analysis. 600bp of COⅠpartial sequences and 481bp of Cytb partial sequences were obtained by using PCR amplification and sequencing. The result revealed that the number of haplotype, haplotype diversities, nucleotide diversities and average nucleotide differences in COⅠand Cytb sequences were 24, 0.729?0.033, 0.00570?0.00325, 3.421 and 28, 0.852?0.016, 0.00645?0.00373, 3.101 respectively. All 7 populations were characterized by higher haplotype diversity and lower nucleotide diversity. A shallow topology of haplotype neighbor-joining (NJ) tree showed no phylogeographic structure among 7 populations. Analysis of molecular variance (AMOVA) and the fixation indices (Fst) of 7 populations showed that the genetic variance mainly came from individuals within populations, and no genetic differentiations between populations were detected from our study. It was supposed that extensive gene exchange between two spawning season cohorts was attributed to continuity of spawning ground and hatching date in temporal and spatial distribution, higher swimming speed of Ommastrephes bartramii individuals and influence of sea currents.