Objective: To investigate protective effects of nicotinamide mononucleotide (NMN) on ethanol-induced DNA damage in L02 cells, so as to provide the evidence for adjuvant therapy of NMN on alcoholic liver diseases. Methods: L02 cells were pretreated with different concentrations of NMN (0, 1, 2, 4 and 8 mmol/L) for 6 h, and then were exposed to 0.4% ethanol for 12 h. The treated cells were divided into the control group, 0.4% ethanol group and different concentrations of NMN groups. Cell viability was analyzed using trypan blue staining for determining the concentration of NMN as a protective agent. The effects of NMN on ethanol-induced DNA damage in L02 cells were evaluated using immunofluorescence detection and reactive oxygen species (ROS) assay. L02 cells were exposed to 0.4% ethanol for 12 h, cultured in a medium containing a protective concentration of NMN, and divided into PBS group and NMN group. Cell viability was detected at 0, 2, 4, 8, 16 and 32 h, and the effects of NMN on repairing ethanol-induced DNA damage were evaluated by alkaline comet assay. Results: The cell viability was lower in 0.4% ethanol group than than in the control group, and was higher in different concentrations of NMN groups than in 0.4% ethanol group (all P<0.05), with no significant difference in the cells viability between 4 mmol/L and higher concentrations of NMN groups and the control group (all P>0.05). Therefore, 4 mmol/L NMN was selected as a protective agent. The cell tail moments, relative immunofluorescence intensities of γH2AX and relative levels of ROS were higher in 0.4% ethanol group than in the control group, and lower in 4 mmol/L and higher concentrations of NMN groups than in 0.4% ethanol group (all P<0.05). The cell viability was increased and the cell tail moment was shortened with the increase of 4 mmol/L NMN intervention time; and the cell viability in 4 h and more of NMN groups were higher, and the cell tail moment were lower than that in PBS group (all P<0.05). Conclusions NMN attenuates DNA damage in a dose-dependent manner and promotes the repair of DNA damage in a time-dependent manner. NMN has a protective effect on ethanol-induced DNA damage in hepatocytes.