The purpose of this study was to investigate the safety of chitosan oligosaccharide and the effects of chitosan oligosaccharide on mercury induced genotoxicity in mice using the micronuclei and chromosome aberration. The micronuclei test was performed by microscopic examination $( imes1,000,;stained;using;a;May-Grunwald;solution)$ after administering 0.01, 0.1, and $1\%(10;mg/mL)$ chitosan oligosaccharide for 7, 60, and 180 days ad libitum in mice. Total micronuclei of 1,000 polychromatic erythrocytes were recorded for each group. There was no difference between the untreated and experimental groups. The intake periods and concentrations of chitosan oligosaccharide did not affect the occurrence of micronuclei in bone marrow cells (P>0.05). The chromosomal aberration test was performed by microscopic examination $({ imes}1,000,;stained;using;a;4\%;Giemsa;solution)$ after administering the same concentration of chitosan oligosaccharide to mice, in $F_1,;F_2,;F_3$ generations and parents. The frequency of chromosomal aberrations was defined as [Ydr=(D+R)/total number of counted lymphocytes]. Similar to the micronuclei test, there was no difference between the untreated and treated groups. These results showed that the intake periods and concentrations of chitosan oligosaccharide did not affect chromosomal aberrations in bone marrow cells (P>0.05). To investigate the effect of chitosan oligosaccharide on mercury-induced chromosome aberration, mice in each condition were supplied with $^{203}HgCl_2$ and chitosan oligosaccharide ad libitum. Chitosan oligosaccharide significantly inhibited $^{203}HgCl_2-induced$ chromosome aberration in mice. Based on the results of this study, it may be concluded that the chitosan oligosaccharide is a nontoxic material that could be used as a suppressor of heavy metal-induced genotoxicity.