The objective of this study was to evaluate the effects of different dosages of Saccharomyces cerevisiae fermentation product (SCFP; Original XP; Diamond V, Cedar Rapids, IA) on lactation performance of mid-lactation dairy cows under heat stress. Eighty-one multiparous Holstein lactating dairy cows were divided into 27 blocks of 3 cows each based on milk yield (23.6 ± 0.20 kg/d), parity (2.88 ± 0.91) and days in milk (204 ± 46 d). The cows were randomly assigned within blocks to one of 3 dietary treatments: 0 (control), 120, or 240 g/d of SCFP mixed with 240, 120, or 0 g of corn meal, respectively. The experiment lasted for 9 weeks, using the first week as adaption period. All data were analyzed through the PROC MIXED program of SAS with the covariance type AR (1) for repeated measures. Means were separated using the PDIFF option in the LSMEANS statement. Linear and quadratic effects of treatment were tested using orthogonal polynomial contrasts. During the experimental period, average daily Temperature-Humidity Index (measured at 08:00, 14:00 and 20:00) was above 72 for 47 out of 56 d (84%). Rectal temperatures tended to decrease linearly (P = 0.07) for cows supplemented with SCFP compared with control cows at 14:30, but were not different at 06:30 (P > 0.05). Dry matter intake was not affected (P > 0.05) by SCFP supplementation. Milk yield increased linearly (P = 0.02) with increasing dosage of SCFP. Feed efficiency (milk yield/dry matter intake) was highest (P = 0.04) for cows fed 240 g/d SCFP. Net energy balance also increased linearly (P < 0.01) with increasing dosage of SCFP. Cows supplemented with SCFP gained (P < 0.01) body weight, while control cows lost body weight. Concentrations of milk urea nitrogen decreased linearly (P < 0.01) with increasing dosage of SCFP, while no difference was observed among the treatments in conversion of dietary N to milk N. In summary, supplementation of SCFP alleviated the negative effect of heat stress in lactating Holstein dairy cows and allowed cows to maintain higher milk production, feed efficiency and net energy balance. Effects of SCFP were dose-dependent and greater effects were observed from higher doses.