The membrane leakage rate and chloroplast ultrastructure were investigated in the fronds of two duckweeds，Lemna minor L．and spirodela polyrrihiza(L_)Schleid，cultured in distilled water and in diferent pH values(5.5，4.5，3.5，2.5)of simulated acid rain for 48 h．The membrane leakage rate and alteration of chloroplast ultrastructure in tw o duckw eeds were enhanced with the decreasing pH value． A diferent sensitivity to acid rain and changing paaem of injured chloroplast morphology were found between two species．In L．minor，the damages of chloroplast and membran e system began at pH 4.5， subsequently， severe plasmolysis， circular dilated chloroplast and the confused structure of grana with numerous vesicles were observed， as pH value of acid rain was dropped down to 3.5．However，in S.polyrrihiza under pH 3.5，less membrane leakage rate (17％) was accompanied by partial break in the outer envelope of chloroplast， while the structures of grana and stroma thylakoid showed no obvious change．The evident damage of chloroplast ultrastructure in S．polyrrihiza appeared at pH 2.5， but no severe plasmolysis and chloroplast swelling were seen． It is postulated that in the freshwater body，S．polyrrihiza may exhibite a stonger ability of existence competition to acid pollution than L．minor，and L． mino r may be used as a sensitive plan t indicator for water body with pH value less than 4．5．