The liver microstructure and ultrastructure of 8-month-old, 20-month-old and 30-month-old Oncorhynchus mykiss were observed by both light and electron microscopy. The results showed that the hepatic capsule thickness varied with age. The parenchyma showed tube-like arrangement of two-hepatocytic plates encircling central venous radially. The hepatocytic microvilli extended from hepatocytes into biliary passages and towards the space of Disse. The hepatic stellate cells (HSC) were located in the space of Disse with irregular shape and angular nuclei and rich in heterochromatin and paranuclear lipid droplets. Cytoplasmic processes radiated randomly from the cell body and round sinusoid. There were free ribosomes, rough endoplasmic reticulum, Golgi complex and mitochondria in the cytoplasm and cytoplasmic processes. There were macrophages in the sinusoid, but Kupffer cells were not evident in the liver. The stromal reticular fiber increased with age. HSC and sinusoid developed perfectly in all age groups. Vessels and bile ducts were observed to scatter without any apparent order within the parenchyma. The portal area was arranged as solitary biliary, biliary-arteriole, biliary-venous and venous-biliary-arteriolar tracts. Structural differences of the portal area between different ages were observed. It was mainly the biliary solitary tube at 8-month old, while there were mainly biliary-arteriole type at 20-month-old and venous-biliary-arteriolar type at 30-month-old. These results indicated that the liver structure varied with in O.mykiss.