Radiation defects-induced plastic flow localization is the origin of loss of ductility in irradiated metals.Defect-free channels (DFCs) are a typical form of strain localization that lead to crack initiation and prema-ture failure.A comprehensive understanding of the DFC dynamics is key to managing radiation boosted property degradation.Despite great research efforts,a clear mechanism of DFC remains unknown.Here,our in-situ tests on irradiated Ni pillars provide a real-time observation of the dynamics of DFCs,includ-ing DFC initiation,extension and thickening.The merging and spreading of dislocation loops serve as an alternative mechanism of dislocation sources that emit massive dislocations and initiate nano-thick DFCs inside the grain.Nano-thick DFCs were formed through chopping up or sweeping away of loops by mobile dislocations.Annihilation of opposite loops and interactions between loops and vacancies accelerate DFC extension.Activation of multiple dislocation sources and dislocation cross-slips are the mechanisms for DFC thickening.