Objective: The purpose of present study was to investigate the effects of condensed molasses fermentation solubles (CMS) on lactation performance, rumen fermentation, nutrient digestibility, and serum parameters of dairy cows. Methods: A total of 75 healthy Holstein cows with the same parity (milk production = 35 ± 2.5 kg, body weight = 570 ± 28 kg) were randomly selected and divided into 5 groups. One group served as control group (CON; no CMS), whereas the other 4 groups were CMS1 (accounted for 1% of the diet), CMS2 (2%), CMS3 (3%), and CMS4 (4%). All cows were fed regularly three times each day at 0800, 1600, and 2400 h. Cows received diet and water ad libitum. The experiment lasted for 60 days. Results: Results showed that the dry matter intake, milk yield, and protein of CMS2 were maximum and higher (p<0.05) than CMS4. The ruminal pH was observed less than 6 in CMS3 and CMS4 groups. No noticeable difference of microbial protein was found between CON and CMS2 groups, while the microbial protein in these groups was higher (p<0.05) than CMS3 and CMS4 groups. The apparent digestibility of dry matter, organic matter, and crude protein in CMS2 group was higher (p<0.05) than CMS3 and CMS4 groups. Compared to CMS3 and CMS4 groups, the CMS2 group increased (p<0.05) the serum concentrations of immunoglobulin G and immunoglobulin M on d 60. Conclusion: Therefore, it is practicable that CMS substitutes for a part of concentrates in lactating cows diets, but higher addition of CMS (more than 3% of the diet) could decrease production performance of dairy cows in the present study.