The particle size (PS) of a diet along with its fiber content is important to maintain optimum rumen function and animal performance. Most studies have focused on the PS of alfalfa and corn silage (CS) with relatively little work on grass silage (GS) PS despite its predominance in the diets of cows in Northern Europe. The objective of our study was to evaluate the effect of GS chop length and diet GS:CS ratio on cow performance and milk composition. Grass was harvested at 2 theoretical chop lengths (short; 10 and long; 44 mm) and mixed with corn silage (GS:CS) at 100:0 or 40:60 (dry matter [DM] basis) of DM in isonitrogenous total mixed rations. Sixteen multiparous Holstein-Friesian cows were used in a 4 × 4 Latin square design with 4 periods each of 28-d duration. Dry matter intake (DMI), milk yield, and milk composition were measured during final week of each period. Composite milk samples from 4 consecutive milkings were extracted and methylated before fatty acid (FA) analysis by gas chromatography. Data were analyzed as a 4 × 4 Latin Square and 2 × 2 factorial design using GenStat 18. A short chop length GS increased (P < 0.04) DMI by 0.9 kg DM/d compared with the long chop length. There was an interaction (P < 0.02) between chop length and GS:CS ratio with a short chop length GS increasing milk yield in cows fed the 100:0 but not the 40:60 GS:CS based diets. There was a tendency (P = 0.09) for a higher milk fat yield (1.2 g/kg) for the long chop length GS. Cows fed the 40:60 GS:CS based diets had higher milk fat concentrations of C18:0 (P < 0.01), C18:2_n6 (P < 0.01) and total polyunsaturated fatty acids (P < 0.02) compared with the 100:0 GS diets. A short chop length GS increased (P < 0.01) milk C18:3_n3 concentration compared with the long chop length. Cows fed the 40:60 GS:CS diets had a 1.5 g/kg higher (P < 0.01) milk protein content. In conclusion, a longer GS chop length reduced DMI and milk yield, but had little effect on milk fat yield and FA content. Including CS in the diet increased DMI and milk performance irrespective of the chop length of the GS.