Granular, needle-, rod-, or whisker-shaped gamma-Fe2O3 nanocrystals have been prepared via controlling the chemical microenvironment of the reaction system under gamma-irradiation. A novel extension-curl-effect model was proposed to explain the effect of the chemical microenvironment on the morphology of gamma-Fe2O3 nanocrystals. The as-prepared gamma-Fe2O3 nanocrystals were characterized by powder X-ray diffraction and transmission electron microscopy. Their magnetic properties were also studied by zero-field Mossbauer spectroscopy, which indicated that different shaped gamma-Fe2O3 nanocrystals could exhibit interesting magnetic behavior. (C) 2003 Elsevier Science (USA). All rights reserved.