Cell ultra-structure of tobacco leaves was studied. The results show that mature mesophyll cells had 8~9 oval chloroplasts, grana lamellae were distinct, and starch grains and a few lipid droplets could be seen. After the leaf senescence, chloroplasts became round and grana lamellae changed into disarray arrangement and blurring. The uniformity of chloroplast grana lamellae,, the numbers and cristae of mitochondria, and the concentration of nucleus karyolymph were different among different mature grade leaves.