Abstract: Effects of Hg 2+ ,varied with concentration,on electron transport activity of photosystem,absorption spectrum,fluorescence emission spectrum at room temperature,polypeptide compositions and ultrastructure of thylakoid membrane of Nuphar pumilum DC.were studied. Hg 2+ inhibited the electron transport activity of PSⅠ and PSⅡ and the effect was stronger on PSⅠ than on PSⅡ.For the Hg 2+ treatment,its absorption peaks were at 430 nm and 680 nm and its fluorescence peak,too,at 685 nm,but the Hg 2+ treatment did not affect its polypeptide composition.Treatment with 10 mmol·L -1 Hg 2+ caused thylakoid swell and stroma reduce in chloroplast.With the time of the treatment extending,thylakoid swelled further and eventually decomposed. As a consequence, stroma further reduced as well.