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[en] The objective of this study is to investigate the microstructural evolution of Y–Ti–O nanoparticles in ferritic 12Cr-ODS steel under in-situ electron radiation. 2 MeV high voltage electron microscope (HVEM) was used for the direct in-situ observation of microstructural evolution at 723 K. The focused beam technique was employed to study the effect of vacancy concentration gradient where the electron beam intensity has a Gaussian distribution. Unlike the typically reported monotonic size change of nanoparticles, under both the focused and uniform beam irradiation, rapid growth and shrinkage were simultaneously observed in each irradiation time step then gradually stabilized. Each nanoparticle did not follow a periodic volumetric change pattern, and this stochastic behavior of Y–Ti–O nanoparticles at the early irradiation stage is first reported in this paper. However, on the contrary to the stochastic volumetric change in each short-time irradiation, from the total volumetric change perspective, the nanoparticles were stable. Thus, the simple total volumetric change of nanoparticles after the irradiation could be inaccurate to fully describe the microstructural evolution of nanoparticles. The shape evolution was observed to investigate the change in morphology and represented by eccentricity and based on different size groups of the nanoparticles, the eccentricity evolution trend was different under the irradiation.