The relationship between diets digestibility and morphology of digestive systems, were studied in local Tanzanian Short Horn Zebu (TSHZ) cattle finished on agro industrial by products. Forty five TSHZ were weighed and randomly allocated to five dietary treatments in a completely randomized design, and nine grazer animals were used as a control group which made a total of fifty four animals. The feedlot animals were fed on hay as a basal diet and supplemented with concentrate, the control group was fed on natural pasture. Animals under Treatments 1 – 5 were fed adlibitum with five concentrates formulated to contain Molasses + Maize meal (T1), Molasses + Maize bran (T2), Maize meal + Maize bran (T3), Molasses + Rice polishing (T4) and Maize meal + Rice polishing (T5). In addition, cotton seedcake, mineral mix, salt and urea were included in all Treatment diets, to meet the requirements for CP and minerals. Digestibility of the diets was measured using Acid Insoluble Ash as a marker. The Morphology of the rumen, duodenum, small intestine and liver from the animals were evaluated after slaughter using ruler and light microscope. Animals under Treatment 2 exhibited significantly higher apparent digestibility of DM (63.9%) and OM (58.8%) than those fed on T1 (62.2% and 55.3%) and lowest values were observed on control group (35.7% and 32.6%) respectively. Higher digestibility values for NDF and ADF (P<0.01) were observed in cattle under T2 (57.4% and 54.8%) and T1 (54.3% and 49.9%) than the other treatments. Rumen papillae length (16.1 mm) and width (3.4mm) for the cattle assigned to Treatment 2 were longest (P<0.01) and widest than those of other Treatments. The villi length (78.5 μm), and crypt depth (29.7 μm) of the small intestine from cattle under Treatment 2 were significantly (P<0.01) longer than those from cattle under the other iii Treatments while the control had the shortest villi (54.2 μm) and narrowest crypt depth (17.5 μm). Cattle under Treatment 2 had heavier carcass (153kg), liver weight (3.86kg) and glycogen level (55.1%) than cattle assigned to the other Treatments. Treatment 5 had the lowest carcass weight (133kg), liver weight (3.03kg) and glycogen level (33.0%). At same time, there was a positive correlation between DM and OM digestibility with carcass, liver weight and glycogen level at (P<0.05), also NDF and ADF digestibility had significant (P<0.05) correlation with carcass and liver weight. Also there was a simple correlation coefficient between papillae (length and width), villi (crypt and depth) with a series of DM, OM, NDF and ADF apparent digestibility (P<0.05). From the above findings, it seems that, for better animal performance, cattle should be fed the well digested feed, which can lead to high morphological improvement. Treatment 2 was the best in categories of digestibility and morphology of digestive system and carcass weight; so it is recommended to be used for beef cattle fattening under feedlot system.IGMAFU Project and family