This study was conducted to evaluate the effects of alfalfa hay (AH) particle size and soybean hull (SH) on physical effective fibre (peNDF), particle size distribution in diets, chewing activity as well as milk composition and production in early lactating dairy cows. Eight lactating Holstein cows averaging 590±33 kg of BW and 47±13 days in milk were assigned in a 4×4 Latin Square design to a 2×2 factorial arrangement of the treatments: alfalfa hay particle size (fine vs. coarse) combined with soy hull (zero or as a substitute for 50% of AH). The peNDF and physically effective factor (pef) values of diets increased by increase in AH particle size and inclusion of SH in diets (P<0.01). Intake of peNDF was increased by increase in the Particle Size (PS) of AH (P=0.08) and SH inclusion (P<0.01) in diets. Milk production was increased by feeding diets containing SH (P=0.04). Milk fat content was increased by increasing PS of AH (P=0.03) and by decrease in SH substitution (P<0.01). Cows fed coarse AH tended to spend more time chewing in comparison with cows fed fine AH (P=0.08). Coarse particles of AH increased total chewing activity based on min/kg of DMI (P<0.05) and tended to increase the eating activity (P=0.09). When chewing activity was expressed as based on NDF intake, ruminating time and total chewing time tended to be increased by coarse AH being fed (P<0.1). Interestingly when chewing activity was reported as based on peNDF intake, the SH inclusion in diet significantly decreased the eating time (P=0.02), total chewing time (P=0.03) and tended to decrease the ruminating time (P=0.08). SH inclusion in the diets increased pH of the feces (P<0.01). With regard to ruminating time and milk fat percentage, the results of the study indicated that fiber in SH can not be physically as effective as that in AH. In diets containing non-forage fiber sources (as SH in this study) using Penn Stat Particles Separator cannot be a reliable tool for an evaluation of some physical properties of diets.