In order to study the effects of particle size on the stability of calcium salt-fatty acids (CSFAs) in rumen, the changes of dry matter degradation rate and fatty acid composition (FAC) of CSFAs with different particle sizes were determined during rumen fermentation. Four lactating Holstein dairy cows installed with permanent fistulas were selected as experimental animals, and were fed a basal diet. A dose of 400 g unscreened CSFAs was mixed in the concentrate per day per cow. CSFAs were screened into four particle sizes (over 12 meshes, 12 to 40 meshes, 40 to 60 meshes and smaller than 60 meshes). Rumen dry matter degradation rate after fermented for 2, 4, 8, 16, 24, 32 and 48 h was evaluated by nylon bag method. Degradation residues were collected at each time point to analyze changes of FAC. The results showed as follows: 1) it was accounted for 40.20%, 50.30%, 4.50% and 5.00% at particle sizes of over 12 meshes, 12 to 40 meshes, 40 to 60 meshes and smaller than 60 meshes, respectively. 2) FAC of the four particle sizes of CSFAs was similar. 3) Particle sizes of CSFAs had extremely significant effects on rumen dry matter degradation rate (P<0.01), the degradation rate of CSFAs over 60 mushes was stable, while CSFAs smaller than 60 mushes were highly degraded. 4) During rumen fermentation, proportions of C16:0 and C18:0 were increased, meanwhile those of C18:1, C18:2 and C18:3 were decreased, which was more obvious on CSFAs smaller than 60 mushes. In conclusion, CSFAs with small particle sizes is not stable in rumen, and the unsatisfied fatty acids are easy to change into satisfied fatty acids. The results indicate that proper manufacture method should be used to obtain CSFAs with more uniform and suitable particle size.