The aim of this study was to use morphometry to investigate the essential organs of sheep fed with non-conventional diets. Twelve male Omani sheep were fed with one of two types of roughages supplemented with a commercial concentrate for 63 days and then slaughtered. The volumes of their kidneys, testes, and livers were measured. Random samples from the organs were fixed in formalin and embedded in paraffin. Random five micrometer sections from each block were stained with hematoxylin and eosin. Images from four random fields were taken from each slide using a digital camera attached to the microscope and then loaded onto a stereology software installed on a personal computer. Morphometric measurements of body organs were taken. Animals fed diets containing tannins had lower final total body weight and lower weights and volumes of their livers, testes and kidneys. There was high variability in the results of intestinal parts measurements, but there was a general trend of the treated animals having higher duodenal and ilial height, villus height, crypt depth and epithelial thickness. The only significant diet effect was higher liver l and lower Vfrac cytoplasm in the control animals than in those treated. The only significant treatment effect on the kidney measurements was a higher tube length/density in treated animals. There were no treatment effects on the testes measurements. The current study indicates that sheep may safely utilize diets containing the levels of polyphenols and tannins used here with no drastic effects on the digestive system and vital organs.