OBJECTIVE To observe the ultrastructural changes of liver tissues on normal rabbit ablated by high-intensity focused ultrasound (HIFU). METHODS A single shot of 1.1 MHz focused ultrasound at an intensity of 500 W/cm(2) with 20-s duration of continuous exposure was applied intraoperatively in normal rabbit livers. Ultrastructural changes of the sonoablated lesion, as viewed by light and electron microscopy, were observed. RESULTS Liver cells at the center of the sonoablated lesion showed irreversible degeneration immediately after HIFU treatment; electron microscopy showed that although the liver cells appeared normal histologically, irregularly shaped cavities of about 0.3-0.5 μm in diameter were present in the cytoplasm. CONCLUSIONS Thermal damages may be the main mechanism of HIFU-induced ablation of liver tissues besides cavitation effect.