The objective of this study was to determine the interactions between alfalfa hay and corn grain particle sizes on nutrient intake, milk yield, milk composition, chewing activity, and ruminal pH of dairy cows. Alfalfa hay was chopped either at 4 cm (long; L) or 1 cm (short; S) theoretical length of cut. Particle size of corn grain was obtained by grinding either at 1 mm (coarse; C) or less than 1 mm (fine; F) mean particle size. Four lactating multiparous Holstein cows were assigned in a 4x4 Latin Square design in a 2x2 factorial arrangement of treatments with 21-day periods. Intakes of DM (DMI) and NDF (NDFI) were not different among treatments averaging 23.6 and 10.6 kg/d. Similarly, yields of milk and milk protein were not different among treatments averaging 22.8 and 0.75 kg/d, respectively. However milk fat yield tended to be higher for C (0.82 kg/d) than F (0.73 kg/d) corn grain. An interaction trend was observed between particle sizes of alfalfa hay and corn grain for milk protein percentage where it tended to be higher for SC (3.31%) compared to SF (3.23%). Chewing activity was not affected by any treatment. Ruminal pH was higher at 9 h of post-feeding for both SF (6.50) and SC (6.36) compared to LF (6.06) and LC (6.17). The data indicate that particle size of alfalfa hay had no effect on milk yield and any milk composition but diets containing C corn grain resulted in a higher milk fat yield.