Abstract: Objective To conduct a comparative study of indocyanine green ( ICG) clearance test and13C- methacetin breath test (13C- MBT) in assessing the liver function in acute liver injury. Methods Forty- eight male Sprague- Dawley rats were randomly divided into control group and 6, 12, 24, 48, and 72 h model groups. The rats in control group were intraperitoneally injected with saline, while the rats in model groups were intraperitoneally injected with D- galactosamine ( 450 mg / kg) to establish an acute liver injury model. The pathological score of liver tissue, ICG retention rate at 15 minutes ( ICG R15) , and parameters of13C- MBT, such as duration to peak of13CO2delta over baseline ( DOB) curve ( T max) , maximum metabolization velocity ( MV max) , cumulative excretion at 120 min ( CUM120) , and maximum DOB ( DOB max) were monitored.Comparison between groups was made by one- way analysis of variance; pairwise comparison of means between samples was made by SNK- q test;correlation between two variables was analyzed by Pearson's correlation test. Results Compared with the control group, all model groups showed significantly increased pathological scores ( P < 0. 05) and significantly decreased MV max, CUM120, and DOB max ( P <0.05) . Pathological score was positively correlated with T max ( r =0.593, P <0.05) and negatively correlated with MV max, CUM120, and DOB max ( r =-0.731, -0.618, and-0.592, P < 0. 05 for all) . Compared with the control group, the 12, 24, and 48 h model groups had significantly increased ICG R15 ( P < 0. 05) , but there was no significant difference in ICG R15 between the 6, 12 h model group and control group ( P > 0. 05) . Conclusion Both13C- MBT and ICG clearance test can be used as the effective tools for monitoring the liver function in acute liver injury.13C- MBT is more sensitive than ICG clearance test in the early diagnosis of acute liver injury.