This study was conducted to investigate the effect of pelletized corn stover replacing alfalfa hay on lactation performance, blood parameters and rumen fermentation dairy cows. Twenty multiparous Holstein dairy cows were blocked based on days in milk (219 ± 49.5 d, mean ± SD) and milk yield (25.4 ± 4.03 kg, mean ± SD), and were randomly assigned into one of 2 dietary treatments with similar concentrate mixture, but different forage ingredients (as DM basis): 15% of pelletized corn stover pellets (CS) or 12.2% of alfalfa hay (AH). Two diets had similar content of net energy for lactation (1.57 vs. 1.58 Mcal/kg DM), but differed in crude protein content (15.8 vs. 16.7 for CS and AH). After feeding for 3 weeks, the rumen fluid and blood samples were collected. The data were analyzed using MIXED procedure of SAS software. The DM intake of diet did not differ (P = 0.72) between 2 diets, with an average of 16.5 kg/d. Milk yield in AH-fed cows was 0.87 kg/d higher than the CS-fed, though the difference was not significant (P = 0.53). No difference was observed between 2 diets in milk protein (3.37 vs. 3.38%, P = 0.97), milk fat (4.15 vs. 4.25%, P = 0.76), lactose (4.78 vs. 4.76%, P = 0.83), total solid (13.7 vs.12.9%, P = 0.16), milk urea nitrogen (16.0 vs. 13.9%, P = 0.19), and somatic cell counts (206 vs. 325 × 10³/mL, P = 0.39). The concentration of plasma glucose was significantly lower in the CS-fed cows than in AH-fed animals (1.82 vs. 2.23 mM, P = 0.02), whereas the other blood parameters (NEFA, BHBA, Ca and P) were not different (P > 0.05). The concentrations of ruminal total and individual volatile fatty acids showed no difference between 2 groups. From the above results, it is indicated that the pelletized corn stover can be used to replace alfalfa hay in the diet for mid-lactation dairy cows without adverse effects on lactation performance and rumen fermentation.