The ultrastructural changes of mesophyll cells in Puccinellia tenuiflora occured during suffering from Na2CO3 stress, were observed using transmission electron microscopy. The results showed that under the normal condition, mesophyll cells arranged loosely, various organelles were morphologically integrated, some starch grains and electron-dense globules (plastoglobulus) were presented in chloroplasts. No alteration of ultrastructure was found by mild Na₂CO₃(2g/L, 4g/L Na₂CO₃) stress. While under moderate Na2CO3 (6g/L, 8g/L Na₂CO₃) stress, the most conspicuous change was the swelling of the thylakoid membranes, disorganization of grana, a marked increase in the number of plastoglobulus, and no starch grains in chloroplasts with the shape of spheroid. The breakdowm of cristae and appearance of certain crystal structure were observed in some mitochondria. The ultrastructure of cell nuclei and vacuole became degraded. Under severe Na₂CO₃(10g/L, 12g/L Na₂CO₃) stress, the chloroplast membrane and the lamellar structure were almost disorganized with a mass of plastoglobulus, mitochondria became disintegrated, the whole mesophyll cells was full of vesicular multicycle-like membrane structures and degraded. These results indicated that the injury in membrane structure of various organelles, especially chloroplast, was closely associated with the eventual death of mesophyll cells.