To determine the role of mitochondrial COⅠgene in classification and identification of species,a total of 39 single individuals from nine species pertaining to two genera of Cynoglossidae in the Yangtze Estuary were barcoded by COⅠ,sequenced and compared with that of other Cynoglossidae species recorded in the Gen Bank. Total genomic DNA was extracted from each scale sample using the classic phenol / chloroform extraction method. Six hundred and fifty base pairs( bp)COⅠfragments were amplified using the primers ’i. e. ’ F1: 5’- TCA ACC AAC CAC AAA GAC ATT GGC AC- 3’,R1: 5’- TAG ACT TCT GGG TGG CCA AAG AAT CA- 3’. Every PCR amplification was performed in a total volume of 50 μL of PCR mixture. PCR products were purified and then sequenced in both forward and reverse directions using an ABI PRISMTM 3730 XL Automated Sequencer. DNA sequences were aligned with clustal W using default parameters. Base composition,variable and parsimony informative sites were determined using MEGA 5. 0. Neighbor- joining( NJ) and Maximum parsimony( MP) phylogenetic trees were constructed for COⅠhaplotypes( Kimura 2 Parameter substitution model,K2P; 1 000 bootstraps pseudoreplications) using MEGA 5. 0. Using the MEGA5. 0 software for statistical analysis,the averaged AT content was greater than the GC content( Tab. 2). The GC content of codon position 1 averaged 53. 8%( 51. 8%-57. 3%),which of position 2 for 42. 0%,and that of position 3 ranged from 28. 1% to 37. 8% in average of 32. 4%( Tab. 4). The transitional pairs( si) was slightly more than the transversional pairs( sv),and the ratio( R = si/sv) was 1. 45( Tab. 3). Analysis of the frequency of amino acids in COⅠgene encoding protein showed that the highest frequency of amino acid was leucine,and the lowest frequency of amino acid was tryptophan( Tab. 5). The average K2 P distances pairwise-species and within-species were 0. 191 and 0. 003,respectively( Tab. 6). The K2 P distance pairwise-species was 63. 7 times of that within-species. According to the MP and