An aquatic fern leaf,Flabellariopteris mii Sun gen.et sp.nov.from the Late Triassic of Western Liaoning,China,is described and proposed as the type species of the new morphortaxon of the Marsileaceae.The fossil leaves are preserved as impression.No fertile and rhizomatic materials are found.The leaves attached at a common point on the top of the petiole are composed of two leaflets joined basally in an opposite arrangement.The petiole is slender.The leaflet is fan-shaped or semi-circular,and commonly divided into four wedge-shaped lobes.Terminal margin of lobes is usually incised,incisions shallow or deep,with bluntly rounded or notched apex.The veins are conspicuously dichotomous,but anastomoses and marginal vein are absent.The unique morphological features,including twoparted thin leaflets,size,shape,veins and slender petiole,indicate that the present specimen represents an aquatic fern,which is the oldest fossil record and the first megafossil evidence assignable to Marsileaceae from the Triassic floras. An aquatic fern leaf, Flabellariopteris mii Sun gen.et sp.nov.from the Late Triassic of Western Liaoning, China, is described and proposed as the type species of the new morphortaxon of the Marsileaceae. The fossil leaves are preserved as impression. No fertile and rhizomatic materials are found. The leaves attached at a common point on the top of the petiole are composed of two leaflets joined basally in an opposite arrangement. petiole is slender.The leaflet is fan-shaped or semi-circular, and commonly divided into four wedge-shaped lobes. terminal margin of lobes is usually incised, incisions shallow or deep, with bluntly rounded or notched apex.The veins are conspicuously dichotomous, but anastomoses and marginal vein are absent.The unique morphological features, including twoparted thin leaflets, size, shape, veins and slender petiole, indicate that the present specimen represents an aquatic fern, which is the oldest fossil record and the first megafossil evidence assignable to Marsileaceae fro m the Triassic floras.