The aim of this study was to assess the ability of ultrasound biomicroscopy (UBM) to diagnose the initial stages of nonalcoholic fatty liver disease (NAFLD) in a rat model. Methods: Eighteen male Wistar rats were allocated to control or experimental groups. A high-fat diet (HFD) with 20% fructose and 2% cholesterol, resembling a common Western diet, was fed to animals in the experimental groups for up to 16 weeks; those in the control group received a regular diet. A 21 MHz UBM system was used to acquire B-mode images at specific times: baseline (T 0 ), 10 weeks (T 10 ), and 16 weeks (T 16 ). The sonographic hepatorenal index (SHRI), based on the average ultrasound image gray-level intensities from the liver parenchyma and right renal cortex, was determined at T 0 , T 10 , and T 16 . The liver specimen histology was classified using the modified Nonalcoholic Steatohepatitis Clinical Research Network NAFLD activity scoring system. Results: The livers in the animals in the experimental groups progressed from sinusoidal congestion and moderate macro-and micro-vesicular steatosis to moderate steatosis and frequent hepatocyte ballooning. The SHRI obtained in the experimental group animals at T 10 and T 16 was significantly different from the SHRI of pooled control group. No significant difference existed between the SHRI in animals receiving HFD between T 10 and T 16 . Conclusion: SHRI measurement using UBM may be a promising noninvasive tool to characterize early-stage NAFLD in rat models.