The aim of this study was to investigate the effects of straw particle size on rumen fermentation, chewing activity and performance of Holstein dairy cows in early lactation. Twelve Holstein dairy cows (114.09± 11.76 DIM, 48.69 ± 3.24 kg milk yield, 600±18 kg BW and 2.66± 0.26 BCS (average± SD)) were used in a replicated Latin square design with 3 treatments and 3 periods in 28 days. Treatments contained 10% wheat straw with different particle sizes: 1) Fine particle size. 2) Medium particle size. 3) Coarse particle size which were 6.9, 13.3 and 22 mm, respectively. Diets were similar in energy and protein concentration and cows were fed adlibitum with a forage concentrate ratio 32 to 68. Geometric mean particle size of rations with fine, medium and large, straw were 3.38, 3.68 and 4.29 mm, respectively. In experimental diets with increasing straw particle size, retained DM on 19 mm increased on the sieve and that of 8 and 1.18 sieve decreased. However, physical effective fiber was not affected by experimental treatments. By increasing the particle length of straw, cows tended to consume large particles (19 mm), especially in the early hours (4 hours), although at the end of the day, the cows receiving small and medium sized straw consumed the same level of large particles. Intake of large straw also took long particles and consumed a large percentage of fine particles (concentrates). The yield of milk production and the efficiency of milk production and the production of milk compounds were not affected by the treatments. Digestion of DM, OM and NDF increased with increasing straw particle size. Feed intake was similar among diets, but eating time linearly increased, and ruminating time tended to decrease. No significant different was not observed for rumen and fecal pH. Blood urea was affected by experimental treatments along with particle size enhancement (p 0.05). The results of this study show that increasing the straw particle size in high concentrate diets did not have a positive effect on yield, but because of the selection activity against long particles (increased concentrate intake), cows tended to spend less rumination as well as had lower fiber digestibility. Key words : Straw Particle Size, PeNDF, Sorting, Dairy Cow Performance.