Objective To investigate the expression of proapoptotic serine protease, Omi/HtrA2, in the heart, kidneys, liver, lungs and spleen of rats at different ages and to explore the correlation between its expression and the aging of liver function. Methods Male SD rats at the age of 3, 9 or 22 months were employed in the study. Their heart function was evaluated by echocardiography, and liver function was tested by automatic biochemical analysis. The expression level and location of Omi/HtrA2 in the above mentioned organs were determined by immunohistochemical staining. The expression of Omi/HtrA2 in the liver was detected by Western blotting. Results (1) Omi/HtrA2 Was highly expressed in the heart, kidneys, liver and lungs of rats from different age groups, and mainly in certain types of cells in different organs. Compared with the young group, its expression was markedly decreased in the liver of rats from aged groups (P＜0.05). But no significant difference was seen in the other organs between the young and aged groups though with a trend of descending (P＞0.05). (2) There were significant differences in the liver function indices, such as the serum levels of aspartate aminotransferase (AST), serum iron (SI), albumin (ALB), and ratio of albumin to globulin (A/G) between the young and aged rats (P＜0.05). The cardiac function indicators, fractional shortening (FS) and left ventricular ejection fraction (LVEF) were in a trend of decreasing in the aged rats compared with the younger though without obvious difference (P＞0.05). (3) The expression level of Omi/HtrA2 was obviously reduced in the middle aged and aged rats than the young ones (P＜0.05). The liver function was also descended with aging. There was a positive correlation of the expression level of Omi/HtrA2 with the level of ALB and ratio of A/G (P＜0.05 ). Conclusion The liver expression of Omi/HtrA2t is positively correlated with liver function in rats.