The objective of this study was to determine the effects of feeding a Saccharomyces cerevisiae fermentation product (SCFP; NutriTek, Diamond V, Cedar Rapids, IA) during the periparturient period on rumen fermentation of dairy cows fed diets differing in starch content during the first 3 wk after calving using a randomized block design. From wk 4 before the expected calving date to 3 wk after calving, 18 ruminally cannulated multiparous Holstein cows were fed diets without (CON; n = 9) or with SCFP (SCFP; n = 9). All cows were fed a common basal diet (1.43 Mcal NE_L/kg DM; 13% starch) before calving. Cows within each treatment (CON or SCFP) were fed either a low- (LS; 21% starch; n = 9) or high-starch (HS; 27% starch; n = 9) diet from wk 1 to 3 after calving. Rumen pH was measured using a submersible continuous ruminal pH measurement system and VFA profile was analyzed by gas chromatography. Data were analyzed using the FIT model procedure of JMP (SAS Institute Inc., Cary, NC). Duration of pH below 5.8 was longer (406 vs. 195 min/d; P = 0.04) for cows fed the LS diets than the HS diets on d 6 after calving. The LS cows tended to have higher molar proportion of acetate (60.6 vs. 58.2 mol/100 mol; P = 0.08) and lower molar proportion of propionate (23.1 vs. 25.2 mol/100 mol; P = 0.10) on d 21 to 23 after calving. Supplementation of SCFP tended to increase the area below pH 5.8 (54.1 vs. 16.7 pH × min/d; P = 0.09) than CON on d 10 to 13 after calving, but tended to increase minimum rumen pH (5.64 vs. 5.44; P = 0.07) and tended to decrease duration of pH below 5.8 (116 vs. 323 min/d; P = 0.10) on d 21 to 23 after calving. These findings suggest that feeding low starch diets during the immediate postpartum period may not necessarily decrease the risk of sub-acute ruminal acidosis (SARA; pH <5.8) of dairy cows fed a low-energy close-up diet, and that SCFP supplementation has inconsistent effects on rumen pH after calving but may reduce the risk of SARA at the end of calving transition period.