A thoroughly study is carried out on an Achnanthidium species from Li River in Wuling Mountains area using light and scanning electric microscopy. The results confirm that this Achnanthidium species is indeed Achnanthidium rostropyrenaicum. A. rostropyrenaicum possesses two key features: the linearlanceolate valve outline with rostrate ends and the slightly convergent or nearly parallel striae distributed at the ends of the raphe valve. The hymens, which were not involved in the original description, also are studied. The hymenes are sievelike. This study expands the knowledge of the morphological structure and geology distribution of A. rostropyrenaicum.