This experiment carried out to investigate the effects of and interactions between marginal change in alfalfa hay particle size and barley or mixed corn and barley in diet on performance of dairy cows. Eight cows (175 days in milk) were assigned to four treatments including fine alfalfa ( FA ) and long alfalfa ( LA ) that were combined with concentrates based on either only ground barley ( GB ; FAGB and LAGB ) or 50% ground barley + 50% ground corn ( GBC ; FAGBC and LAGBC ) in a double 4 × 4 Latin square that arranged in a 2 × 2 factorial design. Diets were fed ad libitum as total mixed ration with a concentrate to forage ratio of 60:40. Averaged NEL predicted, FNDF, CP and NFC were same for all diets and average NDF and ADF were 36.8 and 16.8 for GB and 36.5 and 16.1 for GBC respectively. Geometric mean particle size for FA and LA were 3.43 and 4.33 mm and also for FAGB, LAGB, FAGBC, and LAGBC were 3.6, 3.8, 3.4, and 3.7 mm, respectively . Dry matter intake as other nutrient was greater in GBC than GB (1.34 kg/d) that was not affected by forage particle size. Fiber digestibility was higher for GBC than GB. Barley based diet had more non fiber carbohydrate ( NFC ) digestibility than GBC and with increased alfalfa particle size NFC digestibility was increased and decrease respectively for GB and GBC. The rumen pH at 3 h post feeding and total chewing activity increased as hay particle size increased; instead ruminal pH at 6 h post feeding and total eating activity for GBC was higher than barley. Acetate to propionate acid ratio was tended to decrease with increased hay particle size. Total mean retention time was decrease and passage rate was increased with decreased hay particle size but rumen mean retention time was unaffected from hay particle size. Fractional passage rate and VFA production was not affected from source of grain. Milk production was unaffected, but milk fat to milk protein ratio was increased as alfalfa hay particle size increased. Hay particle size could not affect feces particle size and fermentability; however GBC had higher feces particle size and fermentability. Body weight tended to be high for GBC but total efficiency of energy was 0.52 vs. 0.58 for GBC and GB respectively. Increased of hay particle size or changed in grain source could not affect particle sorting. This study show that increase about 1 mm with the initial 3 mm alfalfa hay particle size may improve animal health and performance and prevent from mild acidosis, particular when barley is dominant grain fed. Keywords: physical fiber, hay particle size, barley, corn, feed efficiency