Gap junctional intercellular communication (GJIC) is an important function of metazoan cells and is believed to play a role of enhancing irradiation-induced bystander responses and have beneficial effects in anti-tumor therapy. This study found that, when primary fibroblast AG1522 cells on confluent were irradiated with a 100 keV/μm carbon-ion beam, the cell responses of micronuclei (MN) formation and G1-phase arrest had a low dose sensitivity effect that was positively regulated by the GJIC. Meanwhile, when neoplastic human salivary gland (HSG) cells were irradiated under the condition of confluent but with poor internal GJIC, MN and G2-phase arrest were induced. Unexpectedly, when GJIC between HSG cells were enhanced by treating of cells with 8-Br-CAMP, these radiation-induced cellular damage was reduced so that cell surviving fraction was slightly enhanced, suggesting that increased GJIC protects HSG cells from lethal radiation damage. Further investigation demonstrated that ROS were involved in the GJIC-enhanced radiation effect on AG1522 cells and nitric oxide contributed to the GJIC-reduced radiation effect on HSG cells.