Abstract: The grain size effect is one of the key factors governing the dynamic mechanical response of metallic materials. In this work, phase transformation iron is selected as the model material, and a series of nanocrystalline polycrystals with identical topology and grain orientation distributions but different grain sizes are constructed to investigate size effects under a fixed grain configuration. Molecular dynamics simulations show that, under high strain rate uniaxial compression, all models undergo the processes of elastic deformation, α→ε phase transition, and high-pressure phase plastic deformation. During the elastic stage, grain boundaries act as a soft layer, leading to lower stresses in the fine grain models than in the coarse grain ones. After the structural phase transition, grain boundaries hinder the plastic development of the new phase, so that the fine grain models exhibit higher stresses than the coarse grain models. At the onset of phase transition, smaller grains possess a lower threshold of phase transition, and the transformed phase in fine grains mainly forms stacking fault structures, whereas twinning structures appear in relatively larger grains. With increasing strain, the disappearance of twinning and the reconstruction of stacking faults are observed in large grains. Under high strain rate tension, shear strain of grain boundary in the large grain models is highly localized, readily forming continuous shear bands that serve as preferred paths of crack propagation. After grain refinement, shear strain of grain boundary gradually evolves into a diffuse mode, and the effective paths of crack propagation are constrained by the network of grain boundaries. The change of grain boundary effects leads to a non-monotonic variation of fracture strength with the grain size.