The objective of this study was to investigate effects of dietary carbohydrate composition with different dietary neutral detergent fiber (NDF):starch ratio by altering the forage to concentrate ratio on milk production and milk composition synthesis in dairy cows. Eight primiparous (146 ± 21.6 d in milk) including 4 rumen cannulated dairy cows were assigned to 4 total mixed rations (TMRs) in a replicated 4 × 4 Latin square design. Each experiment period was 21 d containing 14 d of adaptation and 7 d of measurement. Four dietary treatments were designed in which corn grain was gradually replaced with corn silage and oat hay. The NDF:starch ratios of TMRs were 0.86, 1.18, 1.63 and 2.34 from group T1 to T4, respectively. Dry matter intake (DMI) and milk production was averaged 20.8 kg/d and 31.4 kg/d, and decreased by 21.1% and 14.8% from group T1 to T4 (P < 0.01). Digestibility of dry matter (DM), organic matter (OM), NDF, crude protein (CP) were linearly decreased from group T1 to T4 (P < 0.01), and averaged 69.9%, 72.1%, 50.5% and 72.8%, respectively. However, digestibility of starch showed no difference among groups and averaged 93.7% (P > 0.05). As NDF:starch ratio increased, milk protein content and production, and milk lactose content and production were linearly reduced (P < 0.01), and averaged 3.1%, 0.97 kg/d, 4.95% and 1.56 kg/d, respectively. However, milk fat content was linearly increased from 3.72% to 4.25% from group T1 to T4 (P < 0.01). Quadratic effect was observed on milk fat production (P < 0.01), which increased from 1.23 kg/d to 1.31kg/d from group T1 to T3, and then decreased to 1.20 kg/d in group T4. Therefore, it implied that great attention should be directed at this dietary factor (NDF:starch ratio) in practical diet formulation.