Jaridonin, a novel diterpenoid from Isodon rubescens, has been shown previously to inhibit proliferation of esophageal squamous cancer cells (ESCC) through G2/M phase cell cycle arrest. However, the involved mechanism is not fully understood. In this study, we found that the cell cycle arrest by Jaridonin was associated with the increased expression of phosphorylation of ATM at Ser1981 and Cdc2 at Tyr15. Jaridonin also resulted in enhanced phosphorylation of Cdc25C via the activation of checkpoint kinases Chk1 and Chk2, as well as in increased phospho-H2A.X (Ser139), which is known to be phosphorylated by ATM in response to DNA damage. Furthermore, Jaridonin-mediated alterations in cell cycle arrest were significantly attenuated in the presence of NAC, implicating the involvement of ROS in Jaridonin's effects. On the other hand, addition of ATM inhibitors reversed Jaridonin-related activation of ATM and Chk1/2 as well as phosphorylation of Cdc25C, Cdc2 and H2A.X and G2/M phase arrest. In conclusion, these findings identified that Jaridonin-induced cell cycle arrest in human esophageal cancer cells is associated with ROS-mediated activation of ATM–Chk1/2–Cdc25C pathway.