DNA double-strand breaks (DSBs), from localized DNA single-strand breaks (SSBs), are the most toxic DNA lesions induced by ionizing radiation and are the major actor responsible for radiation-induced cell death. Ionizing radiation-induced DSBs are not only influenced by the dosage of irradiation but are also by whether the radiation source has high- or low-linear energy transfer (LET), such as carbon-ion or X-ray beams, respectfully. The ability to induce DSBs by each source of irradiation can be observed at increasing dosages by agarose gel electrophoresis. In this chapter, we present a technique using agarose gel electrophoresis that provides both a simple and effective way to compare differences behind DNA damage capabilities of ionizing radiation sources of increasing LET and how it can also be a simple method to test certain chemicals for their ability to either increase (radiosensitization) or decrease (radioprotection) the DNA damage associated with each source of radiation in a controlled environment.