Sixty-two mtCOI sequences were determined for 22 medusozoa species sampled from the northern Chinese inshore regions. Two hundred and seven sequences, including 164 additional mtCOI genes from GenBank were compared to test the utility of this molecular marker for species identif ications. The intraspecific variation of the gene ranged from 0 to 7.4% (p-distance), with the average of 0.9% (SD=0.014). More than 93% the pairwise comparisons gave the divergence smaller than 4%. Meanwhile, the counterpart between species within the same genera ranged from 5.4% (Sarsia) to 44.9% (Lensia), with the average of 25.1% (SD=0.118). More than 97% of the variations were larger than 10%. For all the species collected in China, no overlap was observed between the variations within and among species. In conclusion, the barcode gap, which ensured that cox1 sequences can be used as reliable characters for species identification, can be determined in our study. Monophyly of each species was resolved in a neighbour joining tree based on mtCOI genes, which confirmed the efficiency of mtCOI for species identification. The proposals to conserve jellyfish samples were tested. No candidate pro-tocol could keep morphologial keys for species identification for a long time. However, DNA conserver solution was preferred owing to its better performance on maintaining both molecular and morphological features. Furthermore, the tax-onomies of blooming jellyfishes in China were checked using DNA barcodes.