This paper aims to prove the effects of Squalene (SQ) on the cellular toxicity of 5-FU to the mouse which pretreated with SQ and then treated with 5-FU. The results of the group A (treated with only 5-FU) are as follows. The nucleus was destroyed at 24 hours and 48 hours group, however, somewhat repaired at 72 hours group. The dilated inner cavity and the irregular lamellae of the rough surfaced endoplasmic reticulum (RER) were observed continually until 72 hours group. The inner cavity of the smooth surfaced endoplasmic reticulum (SER) were dilated in all groups. However, the destroyed and the normal membrane were observed simultaneously at 72 hours group. The inner membrane of the mitochondria were almost repaired at 96 hours group. The results of the group B (treated with 5-FU and squalene) are as follows. The nucleus was a little influenced by the toxicity of 5-FU at 24 hours and 48 hours, RER were observed to keep the typical lamella structure of cisternae from 24 to 72 hours group, but inner cavity kept on dilating. In SER, inner cavity were also observed to flatten from 24 to 72 hours group. Mitochnodria were always shown normal. All cell organelles were simillar to those of normal groups at 96 hours. Accordingly, it can be said that the treatment of 50 prevents the cytotoxicity of 5-FU on cell organelles of liver cell and that is concerned with the formation of membrane system of cell organelles.