Marine algae are numerous,and often difficult to be identified and classified based on traditional taxonomy due to variable morphological characters and complicated life histories.The development and application of molecular markers make DNA to be a useful tool to differentiate different species and have successfully determined the species identities of many problematic taxa.1733bp nucleotides of 18s rDNA from a marine algae ST3 were determined in this study.The sequences were then used as molecular criteria to investigate the algae identities and their classification.The result showed that ST3 belonged to Thalassiosirales,Bacillariophyta,although ST3 was more closely related to Skeletonema than did Thalassioria,considerable genetic distance between ST3 and Skeletonema existed,so ST3 might be a new genus in Thalassiora.18s rDNA was useful to identify and classify marine algae.