The current study was conducted to investigate the conditions of Isfahan industrial dairy farms in terms of particle size and the supply of physically effective fibre and its effects on physiological responses, health, production and milk component. In this experiment, a total of 15 industrial dairy cattle farms were studied. One group of the high-producing cows (mean 45 ± 5 kg) and mid-lactation (Mean 110 ± 15 days) per farm with cows having more than two parturitions were chosen and studied for a week with daily evaluation of feed particle size, the pattern of feed residual, milk production and composition. Within this week chewing activity was recorded for duration of 24 hrs and rumen pH was measured at the end of the week. The results indicated that farms were different in terms of feed particle size. The differences in geometric mean particle size was up to 2.5 mm and considering the physically effective fiber based on university of Peylvania new recommendations, the differences were up to 3 percent among the herds (P 0.05). Subsequently, these 15 herds were divided into 3 groups based on geometric mean particle size of the feed as fine, medium and coarse particles. It was observed that increasing particle size was associated with an increase in physically effective NDF (P 0.05). As days passed cows were tending to select in favour of fine particles and against coarse particles but no differences between treatments in the selection process of the material according to Peylvania sieves, were detected. However, selection against effective physical fiber increased with particle size increasing (P 0.05). Increase in particle size did not affect the eating behavior of the herd but these herds tended to increase rumination with increasing particle size. Herds in the medium particle size group significantly showed more rumination activity relative to those in fine particle size group (P 0.05). This phenomenon led to a change in the production of milk, milk fat and other milk fat related parameters with varying the particle size, so that, the animals in medium particle size group had the highest performance and the fine particle size group had the poorest performance (P 0.05). The fat to protein ratio was less than one (0.95) in fine particle size group and was significantly lower compared to the other treatments (P 0.05), accordingly it seems that these cows have been in sub-acute acidosis condition. Generally, it could be concluded that based on these results, herds offered feed with a geometric mean particle size of 5.36 mm and physically effective fiber of 26.25 had the best productive performance and health status, and smaller or higher particle size would negatively affect performance and health of dairy cattle. Keywords : feed particle size, physically effective fiber, dairy cattle herd, milk production, milk fat