Summary 　Radiations are rays of electromagnetic waves or particle beams travel through a material with bringing energy. There are ionizing and non-ionizing radiations. Ionizing radiations, such as α-ray, β-ray, γ-ray, X-ray, neutron beam, positron beam, π-meson, heavy-ion beam, and etc., can directly or indirectly ionize and/or excite molecules constituting the material which was irradiated. In living cells, the ionizing radiations can ionize and/or excite mainly water molecules and then several reactive species, such as free radicals and/or reactive oxygen species, etc. can be generated. The most reactive species among them is the hydroxyl radical. Hydroxyl radical can almost indiscriminately oxidize important molecules in cell, such as DNA, proteins, sugars, lipids, etc., to make them malfunctioned (the indirect action). Excess level of such oxidative damages can make the cell and/or the individual died, however; actual effects of low dose radiations are still in progress.