The aim of the study was to investigate the effect of grinding size of corn grains at two starch levels on lactation performance, methane production, and blood metabolites of Nili Ravi buffaloes. Sixteen multiparous buffaloes in early lactation (32 ± 12 DIM; mean ± SE), were enrolled into four treatment groups using a 4 × 4 Latin square design and each period consisted of 21-d. The different experimental diets were LSCG = low starch coarse ground corn, LSFG = low starch fine ground corn, HSCG = high starch coarse ground corn, and HSFG = high starch fine ground corn. The treatments contained fine (FG; particle size (PS) = 1.5mm) and coarse (CG = 3.0 mm) ground corn in high- (HS; starch (ST) = 25%) and low- (LS= 15%) starch (dry matter basis), respectively.  Milk yield was increased by 1.85% with increase in starch level and showed a tendency to increase with ST x PS. In response to ST, lactose and milk protein yield were increased by 4.50%, and 2.42% respectively. The HSFG diet exhibited the highest milk protein and lactose yield. The treatment diets showed no difference in milk fat contents and yield. Glucose was higher in FG compared to the CG diets. Gross, metabolic, milk nitrogen efficiencies and enteric methane production was decreased by feeding high - starch diet. Body weight increased with increase in starch level. Diets with 25 %starch level improved milk production, protein, lactose yields and decreased methane production. Starch level had a more pronounced effect than particle size, however, interaction was not observed between starch and particle size for any parameter.