Granular, needle-, rod-, or whisker-shaped γ-Fe 2 O 3 nanocrystals have been prepared via controlling the chemical microenvironment of the reaction system under γ-irradiation. A novel extension-curl-effect model was proposed to explain the effect of the chemical microenvironment on the morphology of γ-Fe 2 O 3 nanocrystals. The as-prepared γ-Fe 2 O 3 nanocrystals were characterized by powder X-ray diffraction and transmission electron microscopy. Their magnetic properties were also studied by zero-field Mössbauer spectroscopy, which indicated that different shaped γ-Fe 2 O 3 nanocrystals could exhibit interesting magnetic behavior.