Extracts of Thai medicinal plants have been reported to be natural sources of antioxidants which can inhibit free radicals and have potential for radiation protection. In this study, we investigated the efficacy of five medicinal plant extracts, cinnamon, ginger, white kwao krua, black pepper, and moringa compared to gallic acid, which served as a positive control in the protection of plasmid pET-15b DNA after exposure to gamma irradiation. pET-15b plasmid DNA was irradiated by gamma radiation at 100 Gy with various concentrations of medicinal plant extracts and DNA damage was measured by using gel electrophoresis. The results showed that exposure of pET-15b plasmid DNA to 100 Gy of gamma radiation induced DNA strand breakage and plasmid DNA alteration from a supercoiled to open circular form. Extracts of moringa, cinnamon and ginger significantly reduced plasmid DNA damage by gamma radiation. While black pepper and white kwao krua extracts did not show observable radioprotective effects. The presence of moringa, cinnamon and ginger extracts during irradiation can protect pET-15b plasmid against radiation-induced DNA damage.