This research was conducted to determine the effects of chitosanoligosaccharide on liver poisoning induced by cadmium (Cd). Three groups of mice were used in this research. The first group was only injected with cadmium (5.0 mg/kg; i.p.) (group Cd) and the second one with cadmium and chitosanoligosaccharide (0.5% solution) at the same time (group Cd+Chi). The third one which had already been injeted with chitosanoligosaccharide (0.5% Solution) aweek before (group Ch7+Cd) was used. In order to investigate the inhibitory action of chitosanoligosaccharide on liver damage, enzyme activity in serum, glutathione peroxidase (GSHPx) activity and glutathione reductase (GR) activity were relatively measured. In addition, histological observations were made to determine the morphologic injury of liver tissues. As the result of enzyme activity in serum, the activity of aspartate aminotransferase (AST), alanine aminotransferase (ALT) and lactate dehydrogenase (LDH) in chitosanoligosaccharide-injected groups Cd+Chi and Chi7+Cd was lower than in group Cd. GSH-Px activity was sharply increased in groups Cd+Chi and Chi7+Cd compared to group Cd. GR activity was conspicuously decreased in groups Cd+Chi and Chi7+Cd compared to group Cd. As the result of light microscopic observation, liver cell necrosis caused by cadmium poisoning was obseved in liver cells. The finding of group Cd+Chi and Chi7+Cd was similar total on of normal groups. As the result of electron microscopic observation, mitochondria in group Cd showed a severe swelling phenomenon, RER fragment and ribosome dropout. However, in groups Cd+Chi and Chi7+Cd, mitochondria wiht high electron density were distributed and RER forming a typical lamellae with ribosome was observed. From these results, cadmium toxicity on rat liver tissues could be lessened by chitosanoligosaccharide.