Non-alcoholic fatty liver disease (NAFLD) is the most widespread chronic liver disease, which is caused by lipid overload. It is related to food with a high carbohydrate and fat content - the Western pattern diet, which is today progressively becoming more common. The purpose of the study was to find out whether the structure and ultrastructure of liver cells of animal fed with Western diet differs from the structure and ultrastructure of cells of the control group. We took mice as experimental models, the first half of them has been fed with the Western diet and the second half with a standard diet ad libitum. After eight weeks, animals were killed and a small amount of liver tissue has been obtained from them by laparoscopy. Thin and ultra-thin mouse liver slices were then analyzed with microscope. The analysis has shown that the structure of liver cells of mice fed with the Western diet is different from cells of the control group of mice. Liver tissue of mice fed with Western diet contained large amount of lipid droplets. We noticed two types of steatosis, micro and macro vesicular steatosis. By examining the liver cells structure of mice fed with Western diet we noticed changes in shape and size of mitochondria. In many cases we also noticed condensed matrix, extended cristae, autophagy and some other structures that indicate a liver cells necrosis.