Recent development of DNA sequencing technologies has enabled successful construction of thousands of mitochondrial genomes. Nevertheless, only 33 mitochondrial genomes have been reported for species in Bacillariophyta, which includes harmful algal bloom (HAB) species. In this study, we determined the complete mitogenome of the Bacillariophyta HAB species Odontella regia. For comparison, we also constructed the mitogenome of Lithodesmium undulatum, another Bacillariophyta species. Both strains were isolated in the Jiaozhou Bay, an epitome of China’s coastal ecosystem and an ideal site for HAB research. These two mitogenomes were characterized as 37,617 bp and 37,057 bp circular-mapping molecules with AT content of 73.4% and 75.3%, respectively. The phylogenetic tree based on 32 protein-coding genes of the mitogenome encoded in 35 Bacillariophyta species revealed that the class Mediophyceae consisted of multi-phyletic clades. While O. regia formed an independent clade, L. undulatum was closely related to Thalassiosira pseudonana and Skeletonema marinoi. Synteny comparison of O. regia and L. undulatum mitogenomes and mitogenomes of three closely related species displayed substantial differences among lineages in Mediophyceae by a series of gene translocations and/or inversion with many conserved gene blocks such as rpl2-rps19-rps3-rpl16-atp9. These analyses suggested a complex evolutionary relationship in Mediophyceae in which the mitogenome of O. regia was the least conservative compared with the mitogenomes of four other Bacillariophyta species. Further studies are needed to clarify detailed phylogenetic relationships in Bacillariophyta.