The current study was carried out to investigate the effect of physical form of the starter and forage on blood parameters, histological alterations and pathogenic bacteria population of small intestine in Holstein calves. This experiment was conducted in completely randomized design with 4 treatments and 13 replications for 70 days. Dietary treatments consisted of mash starter, multi particle starter, pellet starter and mash starter plus 10 percent alfalfa hay. To evaluate blood parameters, calves were bled from the jugular vein on days 25, 50 and 70 of experiment. Also, on days 35 and 70 of trial, fecal samples were taken from the rectum in sterile containers and were cultured on the Eosin Methylene Blue agar medium to enumerate Escherichia coli and Salmonella bacteria. At 70 days of age, three calves from each treatment were randomly sacrificed and then liver and small intestine samples (the part of the duodenum, jejunum and ileum) were prepared and fixed for histological examination; after that, they were cut and colored to examine by light microscopy. Besides, pH value of intestinal content was measured. The results showed a significant difference between treatments in terms of blood parameters including glucose, blood urea nitrogen (BUN), triglyceride, cholesterol, LDL, HDL, globulin, total protein, T3 and T4 concentration as well as the activity of aspartat aminotransferase (AST), alanine aminotransferase (ALT; P 0.05). The Highest concentration of glucose, triglyceride, cholesterol, HDL and the lowest LDL concentration was obtained in calves receiving mash starter with 10% alfalfa hay. White and red blood cell counts were not significantly different between treatments. The most and lowest numbers of Escherichia coli and Salmonella was belonged to pellet starter and mash starter plus 10% alfalfa hay, respectively. Additionally, pellet starter had the highest content of liver fat and the worst hepatic histological integrity. Only thickness of muscularity layer was significantly affected by among the histological parameters of the intestine (P 0.05) and the highest and the lowest thickness was assigned into mash starter plus 10% alfalfa hay and pellet starter, respectively. Intestinal pH value was not significant among dietary treatment. Only the relative weight of the lungs was significant among the relative internal organ weight. Moreover, the highest and lowest the relative weight of liver was related to mash starter plus 10% alfalfa hay and pellet starter, respectively. In general, results of this study indicated that the physical forms of starter affect metabolic parameters in blood and liver, population of gram negative bacteria and intestinal histological parameters and also, the presence of 10% alfalfa hay plus mash starter had the most effect on improvement of calves performance and health via improving metabolic parameters in blood and liver and reducing the enumeration of pathogenic intestinal bacteria. Keywords : Starter - physical form - Blood parameters- intestinal histology - Escherichia coli bacteria – Liver – intestinal pH - weight of internal organs