Nutrient-diluted diets, with lower protein and higher fiber contents, can reduce the cost of food. The present study evaluated the effect of diets with protein and fiber contents below the recommended standard on the growth of male Muscovy ducks (Cairina moschata Linnaeus, 1758) from 28 to 77 days of age. The proximal chemical analysis determined protein and fiber contents of 8.4 and 3.0% for diet 1 and 14.8 and 6.3% for diet 2, respectively. Diet 1 resulted in decreased weight gain, trends for lower carcass yield and liver weight, and increased gut size compared to diet 2. Abdominal fat deposition was not influenced by the diets. The excessive decrease in protein would lead to the appearance of nutrients that limit the synthesis of body proteins, resulting in lower weight gain and yield of the carcass. The trend of lower liver weight and shorter intestinal length in ducks that consumed high-fiber diets suggest less fat deposition in the liver and decreased intestinal absorption area due to the effect of fiber, respectively.