Objective To explore the mechanism of human umbilical cord mesenchymal stem cell (HUC-MSC) alleviating ischemia-reperfusion injury (IRI) of liver cells through mitochondrial transfer. Methods Normal human liver cell line L02 was divided into the blank control group, oxygen-glucose deprivation (OGD) group, experimental control group, and L02 and HUC-MSC co-culture group (L02+HUC-MSC group). L02+HUC-MSC group was further divided into 10:1 co-culture subgroup (group A), 4:1 co-culture subgroup (group B), 2:1 co-culture subgroup (group C), 1:1co-culture subgroup (group D) and 1:2 co-culture subgroup (group E) according to different co-culture ratio of L02 and HUC-MSC. The apoptosis rate and relative reactive oxygen species (ROS) level of L02 cells were detected by flow cytometry. The MitoTracker positive rate of L02 cells was detected by flow cytometry. The mitochondrial transfer from HUC-MSC to L02 cells was observed by laser confocal microscope. Results The apoptosis rate and relative ROS level of L02 cells in the OGD group were significantly higher than those in the blank control group (both P < 0.05). Compared with the OGD group, the apoptosis rates of L02 cells in group B, C, D and E were significantly decreased (all P < 0.05), and the relative ROS level of L02 cells in group E was significantly declined (P < 0.05). The MitoTracker positive rate of L02 cells did not significantly differ between group A and experimental control group (P>0.05), whereas the MitoTracker positive rates of L02 cells in group B, C, D and E were significantly higher than that in the experimental control group in a concentration-dependent manner (all P < 0.05). Under the laser confocal microscope, mitochondrial transfer fromHUC-MSC to L02 cells could be observed through tunneling nanotube (TNT). Conclusions HUC-MSC may alleviate cell apoptosis and reduce ROS level of liver cells after IRI via direct mitochondrial transfer between cells.