Ionic liquids (ILs) have been widely applied in chemical industry due to their unique properties. There is mounting evidence that they are toxic to various living organisms. Thus, it is crucial to develop methods to efficiently degrade and mineralize ILs to reduce their potential risks to the environment. In this work, the degradation of 1-butyl-3-methylimidazolium bromide ([C4MIM]Br), an imidazolium-based IL, was investigated in aqueous solution via Co-60 gamma-ray irradiation. Results show that [C4MIM] Br could be effectively degraded by gamma-ray irradiation. The irradiation-induced degradation of [C4MIM] Br fitted well to the pseudo-first-order kinetics model. The kinetic constant (k) increased linearly with the increase of irradiation dose rate (R-2 = 0.942) and solution pH value (R-2 = 0.974), and decreased with the increase of [C4MIM] Br concentration. Fourteen intermediates were detected using a liquid chromatography-mass spectrometry analysis. Based on these intermediate products, the degradation pathways and mechanisms of [C4MIM] Br were proposed. The free radicals (e.g., aqueous electron (e(aq)(-)), hydroxyl radical (center dot OH), and hydrogen atom (H center dot)) generated by gamma-ray irradiation showed synergistic effects in the degradation and mineralization of [C4MIM] Br. Moreover, the toxicity assessment using C. elegans and Chlorella pyrenoidosa as model organisms demonstrates that sufficient gamma-ray irradiation could effectively reduce the potential risks of [C4MIM] Br and its intermediates to the environment. These findings of this study may offer an efficient and environmentally friendly approach for effectively removing ILs and other persistent organic pollutants.