This experiment was conducted to investigate the effects of different levels of dietary NDF in diets rich in plant oil (sunflower and soybean) on feeding behavior, rumen parameters, milk yield and components of mid-lactating Holstein dairy cows. Four primiparous (BW: 525 ± 30 kg; days-in-milk: 103 ± 6) and 4 multiparous (BW: 587 ± 88 kg; DIM: 99 ± 12) cows were used in a 4 × 4 replicated Latin square design with 21-d experimental periods. Cows were received 1 of 4 dietary treatments: (1) 31% fiber and no supplement plant oil, LFNO; (2) 31% fiber with 3% supplement plant oil, LFHO; (3) 35% fiber with 3% plant oil, MFHO; (4) 39% fiber with 3% plant oil, HFHO. Daily dry matter intake (DMI), milk yield and composition, blood metabolites, dry matter digestibility, rumen fluid characteristics and BW variations were determined. There was no significant different in milk production among diets. Milk fat (P < 0.01) and protein (P < 0.05) concentrations were significantly affected by treatments. Dry matter intake and DM digestibility were higher (P < 0.01) in LFHO diet. The cows fed HFHO had higher NDF digestibility (P < 0.01). Total VFA and acetate concentration were greater for HFHO diet and propionate concentration was greater for LFHO diet (P < 0.01) than the others. Rumen fluid pH was increased by increasing dietary NDF concentration (P < 0.01). Plasma insulin (P < 0.01) and cholesterol (P < 0.05) concentration were affected by diets. Chewing activity was positively affected by increasing concentration of dietary NDF (P < 0.01). This study showed that diets rich in plant oil and low concentration of NDF induce the milk fat depression in mid-lactating cows, and with increasing dietary NDF concentration severity of milk fat depression can be alleviated.