The fat bodies of cabbage worm (Pieris rapae) and silk worm (Bomyx mori) during metamorphosis was comparatively studied by electron microscope. 1. Cell oranelles: Golgi apparatus were not observed in both species. It is observed that RER of cabbage worms initiate to degenerate in prepupa stage with complete degeneration at adult stage, while that of silk worms shows similar degenerative pattern. However, mitochondria of cabbage worms are transformed into autophagic vacuole from prepupa stage until adult stage whereas those of silk worm shows a decrease in number in prepupa stage but maintains a certain level until adult stage. 2. Storage substance in cell: Lipid droplets in cabbage worms were observed to increase in numbers during larval stage but afterward decrease in number with an enlargement in size. However immediately after their pupal stage, they almost disappear. On the contrary lipid droplets in silk worms show rather increase in number until adult stage. Protein storage granules in bothspecies were arised from autophagic vacuoles(lysosome) . Fat cells of cabbage worm in adult stage turn out to be residual bodies which last until final stage, but those of silk worm rapidly decrease. Glycogen particles in both species reach maximum at last larval instar and thee gradually decrease thereafter. 3. Fat body sheath: The average width of fat body sheath was measured to be $0.2{mu}m$ and $0.6{mu}m$ and surface of fat cells adjacent to fat body sheath in silk worm is heavily infolded.