The structure and organization of mitochondrial genomes of 14 marine bivalves and two freshwater bivalves were analyzed using comparative genomics and bioinformatics methods. The results showed that the organization and gene order of the mitochondrial genomes of these bivalve species studied were different from each other. The size, organization, gene numbers, and gene order of mitochondrial genomes in bivalves at different taxa were different. Phylogenetic analysis using the whole mitochondrial genomes and all the coding genes showed different results-- phylogenetic analysis conducted using the whole mitochondrial genomes was consistent with the existing classification and phylogenetic analysis conducted using all coding genes not consistent with the existing classification.