Abstract: Najas major Allioni is a submersed monocotyledonous plant of the family Najadaceae. It serves as a natural food of the grass carp. In 1978 and 1979, this macrophyte was predominant over other submerged vascular plants in the Tanglinhu area of the Donghu Lake,Wuhan. The plant dies off every winter. The purpose of this study is to understand the morphological degradation during detritus formation, so as to provide knowledge for the recognition and quantification of organic detritus at various areas in the Lake.Under laboratory condition, the observed morphological changes may be referred to the following phases: 1. Critical or Premortal Stage, 2. Stage of color change in the nucleus, 3.Stage of cytoplasmic plasmolysis, and 4. Stage of jellylike consistency. The authors take the stage of color change in the nucleus as the initial step of detritus formation. Besides laboratory observation, natural condition of the detritus suspended in water and deposited as sediment in the Tanglinhu area has also been investigated.