The effects of alfalfa hay particle size of goat diets on eating behavior, ruminal pH, and nutrient digestion kinetics were investigated in the study. The experiment was designed as a 4×4 Latin square using 4 goats ［(40.0±2.2) kg BW］ with ruminal cannulas. Diets were chemically similar but varied in forage particle size. The results showed that increasing alfalfa hay particle size altered particle distribution of diets, increased physically effective fiber content of diets, but reduced dry matter intake. Increasing alfalfa hay particle size increased mean ruminal pH and reduced the duration in which pH was lower than 6.0 and 5.6 by 13.8 and 3.2 h/d, respectively. Effective ruminal disappearance of alfalfa hay NDF tended to be increased linearly from 22.61% to 27.74% with increasing forage particle size. Feeding short alfalfa hay tended to increase the disappearance rate and effective disappearance of concentrate DM in rumen (P<0.10). Goats sorted for long particles instead of short ones. This study indicates that increasing alfalfa hay particle size can improve ruminal pH, alleviate subacute ruminal acidosis, and increase fiber utilization. However, it can also reduce dry matter intake if the alfalfa hay is too long.［Chinese Journal of Animal Nutrition, 2010，22（5）:1293-1300］