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In this Letter, we introduce conical-scanningdark-field imagingin four-dimensional (4D) ultrafast electron microscopy to visualizesingle-particle dynamics of a polycrystalline ensemble undergoingphase transitions. Specifically, the ultrafast metal–insulatorphase transition of vanadium dioxide is induced using laser excitationand followed by taking electron-pulsed, time-resolved images and diffractionpatterns. The single-particle selectivity is achieved by identifyingthe origin of all constituent Bragg spots on Debye–Scherrerrings from the ensemble. Orientation mapping and dynamic scatteringsimulation of the electron diffraction patterns in the monoclinicand tetragonal phase during the transition confirm the observed behaviorof Bragg spots change with time. We found that the threshold temperaturefor phase recovery increases with increasing particle sizes and wequantified the observation through a theoretical model developed forsingle-particle phase transitions. The reported methodology of conicalscanning, orientation mapping in 4D imaging promises to be powerfulfor heterogeneous ensemble, as it enables imaging and diffractionat a given time with a full archive of structural information foreach particle, for example, size, morphology, and orientation whileminimizing radiation damage to the specimen. [ABSTRACT FROM AUTHOR]