Forty two male Holstein calves (46 ± 3.0 kg) were used to evaluate the effect of alfalfa hay (as a physical factor) inclusion at different levels and sodium-propionate (as a chemical factor) supplementation on intake, average daily gain (ADG), feed efficiency, weaning age, skeletal growth, fecal score and rumen development in dairy calves. This experiment was conducted as complete randomized design with a 2 × 3 factorial arrangement. Dietary treatments consisted of: 1) concentrate only (Control); 2) concentrate plus 5% sodium-propionate (Control + Pro); 3) concentrate + 5% forage only (5 % F); 4) 5% forage + 5% sodium-propionate (5%F+ Pro); 5) concentrate + 10% forage only (10 % F); and 6) 10% forage + 5% sodium-propionate (10% F+ Pro). All calves received 4 L of colostrums within 12 h of birth. Calves, initially less than 3 days old, were housed in individual pens through 70 days. Dietary treatments were provided ad-libitum in addition to constant amount of milk (4 kg/head/day). Concentrate was provided as meal form and alfalfa hay was chopped with geometric mean particle size between 2.5-2.8 mm. The base of weaning was consumption of 1 kg solid feed during 3 consecutive days. Nine calves from selected treatments (Control, 10% F, and 10% F + Pro; 3 calves per treatment) were euthanized at 70 d of age. Tissue samples were taken from different part of the rumen include (A) caudal portion of the caudal ventral blind sac; (LB) left side caudal dorsal sac; (LC) left side cranial dorsal sac; (LD) left side cranial ventral sac; (LE) left side ventral portion of caudal ventral blind sac. All data were analyzed using the mixed linear model (PROC MIXED) of SAS (SAS 2003).The covariance structure was modeled using an auto regression structure within animals and random effect between animals. Alfalfa hay supplementation significantly increased dry matter intake and average daily gain. However the effects of propionate or propionate × forage interaction were not significant. Propionate and propionate × forage did not significantly affected feed efficiency, on the other hand, alfalfa hay feeding reduced feed efficiency (P 0.02). Weaning age was decreased significantly with forage supplementation in comparison to propionate supplementation (45 d vs. 61 d, respectively). Fecal score were not affected by treatments. Rumen wall in calves fed alfalfa hay had thinner keratin layer with stronger muscles layer compared with control group. However papilla length and other rumen characteristics (such as papilla width, epithelium thickness, rumen wall thickness) were significantly affected by 10% F. Propionate and propionate× forage interaction did not affect rumen parameters. Overall, results of present study indicate that adding forage in the form of chopped alfalfa hay positively influenced intake, ADG and microscopic appearance of the rumen epithelium and rumen wall as well as decrease in weaning age. In addition, rumen epithelium in