Herbal preparations used in the treatment protocols of metabolic disorders are highly effective and less toxic than chemically synthesized ones. Melissa officinalis is a plant that is an example of phytotherapy and functional nutrition, which is associated with many physiological and functional effects, including antioxidant, sedative, anti-inflammatory, antibacterial, antifungal, hypoglycemic and anxiolytic. The purpose of this work was to determine the effect of Melissa officinalis dry leaves on the morpho-functional state of liver during the consumption of high-fat diet by rat. The study was designed with two groups of white laboratory rats (n=6) while control group of animals consumed a high-fat diet for 30 days. The experimental group diet was additionally supplemented with 5% of Melissa officinalis in the form of dry crushed herb. Absolute and relative liver mass, the liver histostructure, and blood biochemical parameters were analyzed to estimate the main characteristics of the liver functional activity. There was established that a high-fat diet caused the progress in fatty liver disease, which was manifested by large droplet vacuolization of hepatocyte cytoplasm. The supplementation with Melissa officinalis did not stop this process, but reduced the intensity of the manifestation (small droplet cytoplasm vacuolization). The suplementation of Melissa officinalis into a high-fat diet caused an increase in absolute and average daily body weight gain, weight of the liver, significantly affects lipid metabolism, the changes in blood enzymes activity, causes a decrease in the blood glucose and total bilirubin level. Thus, Melissa officinalis can potentially be used as a dietary and phytotherapeutic agent.