Effects of grinding corn grain with different particle size and steam flaking on feed intake, milk production, chewing behavior, sorting activity, ruminal fermentation, digestibility, cost over income of feed were evaluated using eight multiparous Holstein cows in a duplicated 4 × 4 Latin square design with 21-d periods. Cows were in mid-lactation (46.6 ± 3.5 milk production and 101 ± 10 days in milk) with an average body weight of 616 ± 58.4 kg. Experimental diets consisted of 36% forage (corn silage and alfalfa hay) and 64% concentrate. Treatments were ground corn with 2, 3 and 4 screens and steam flake corn (density= 410 g/L). Average THI in experiment period was 70.46 and cows were in heat stress condition. Steam flaking vs. grounding corn increased proportion of the particles retained on the 8 ( P 0.001) and 1.18 ( P = 0.002) mm sieve of the and decreased the particles retained on the pan ( P 0.001). Increasing screen diameter caused a linear increase in the proportion of particles retained on the 1.18 mm ( P 0.001) sieve while the proportion of particles retained on the pan ( P 0.001) reduced linearly. peNDF 8 ( P 0.001) and peNDF 1.18 ( P 0.001) increased with steam flaking vs. grounding corn. Dry matter intake, milk production, 3.5% FCM, ECM, and the production of lactose and fat, as well as percentage of lactose and protein and MUN were unaffected by treatments. Rumen pH ( P = 0.003) and milk fat percentage ( P = 0.007) were greater in steam flaked corn than ground corn. Milk fat percentage tended to increase linearly with increasing corn particle size ( P = 0.07). Propionate concentration tended to decrease with steam flaked vs. ground corn ( P = 0.1). Acetate and butyrate concentration, acetate to propionate ratio and ammonia nitrogen concentration were unaffected by treatments. Feed efficiency based on FCM/DMI ( P = 0.02), ECM/DMI ( P = 0.04) were greater in steam flaked corn vs. ground corn. Dry matter and organic matter digestibility in ground corn with 2 screen and steam flaked corn was greater than other treatments. Time spend eating per minutes a day tended to decrease with steam flaked vs. ground corn ( P = 0.07). Time spend eating per kilogram peNDF 8 ( P 0.001) and peNDF 1.18 ( P 0.001) intake was lower in steam flaked corn than ground corn. Rumination activity and total chewing time were unaffected by treatments. Body weight, BCS and back fat thickness were unaffected by treatments. Ground corn with 3 screen reduced respiratory rate ( P = 0.009). Steam flakeing led to increased price per kilogram of ration but, income among treatments were not different. However, income was 1700 rials (per day) higher in 4 screen than steam flaked corn. Generally, grinding corn grain with 4 screen sizes is recommended. Keywords : Corn grain, Particle size, Steam flaking, Processing, Performance, Dairy cows