The objective of this study was to evaluate the effects of various dosages of Saccharomyces cerevisiae fermentation product (SCFP; Original XP; Diamond V) on lactation performance, rumen microbial communities and rumen fermentation in mid-lactation dairy cows fed a total mixed ration containing low quality forages. Eighty multiparous Holstein dairy cows were blocked based on days in milk (180 ± 45 d), parity (3.24 ± 1.06) and milk yield (26.6 ± 0.79 kg/d), and were randomly assigned to one of 4 treatments. Treatments consisted of 0, 60, 120, or 180 g/d of SCFP per head mixed with 180, 120, 60, or 0 g of corn meal, respectively. The forage in the basal diet was comprised (DM basis) of 15% corn stover, 7% Chinese wild ryegrass, and 17.3% corn silage. The experiment lasted for 10 weeks, with the first 2 weeks for adaption. The data were analyzed using the PROC MIXED program of SAS with the covariance type AR (1) for repeated measures. Means were separated by using the PDIFF option in the LSMEANS statement. Orthogonal polynomial contrasts were used for linear and quadratic effects testing. Dry matter intake decreased linearly (P = 0.02), while milk yield increased linearly (P = 0.05) with the increasing dosage of SCFP, resulting in improved (P < 0.01) feed efficiency compared with the control. Contents of milk components were similar (P > 0.05) among the treatments. Neither rumen pH (P = 0.70) nor ammonia-nitrogen concentration (P = 0.82) was affected by SCFP supplementation. Increasing dosage of SCFP linearly increased (P < 0.05) concentrations of ruminal total volatile fatty acids, acetate, propionate and butyrate, with no difference (P > 0.05) in molar proportion of individual acids. The population of fungi and cellulolytic bacteria increased linearly (P < 0.01), but those of lactate-utilizing and lactate-producing bacteria decreased linearly (P < 0.01) with increasing dosage of SCFP. The estimated microbial protein yield in the rumen increased linearly (P < 0.01) in response to SCFP supplementation. In summary, supplementation of SCFP improved the lactation performance of dairy cows consuming diets containing low quality forages by optimizing rumen fermentation through manipulating rumen populations. Effects of SCFP were dose-dependent and greater effects were observed from higher doses.