Abstract: This article reports on the following structural changes of Trichomonas vaginalis under the action of mandelic amide in vitro observed by TEM: 1. cytoplasmic membrane was destroyed; 2. nuclear membrane of the cell disappeared; 3. granular endoplasmic reticulum and free ribosome in the cytoplasme all disappeared; 4. chromatic granules highly electron-dense, and their membrane disappeared. The results suggested that these structural changes might be attributable to the effect of mandelic amide.