Synthesis of uniform magnetic nanocrystals with tunable shape is valuable to investigate the microwave absorption properties that depend closely on the shape and size. In this study, we utilize an efficient method to synthesize nano -sized iron oxide nanocrystals with different shapes through thermal decomposition of Fe(acac)(3) in oleylamine. While the spherical Fe3O4 nanocrystals display a typical superparamagnetic behavior at room temperature, the triangular nanoplates exhibit a blocking behavior at an unexpected high temperature. The antiferromagnetic-ferrimagnetic core -shell structure of Fe0@-Fe3O4 nanocubes presents exchange bias behavior. We also investigate the high frequency properties of all samples by a network analyzer. Compared to spherical and cubic shapes, the triangular Fe3O4 nanoplates exhibit significantly enhanced microwave absorption performance in terms of strong reflection loss and wide bandwidth. Moreover, the triangular Fe3O4 nanoplates have obvious dielectric and magnetic resonance behaviors responding to the microwave at the frequency range of 2-18 GHz. The dielectric and magnetic resonance behaviors may be derived from the interface polarization and exchange resonance. The minimum reflection loss of triangular Fe3O4 nanoplates reaches 32.1 dB at 11.7 GHz and the bandwidth less than 10 dB is from 10.6 to 133 GHz at a thickness of 2.5 mm. (C) 2017 Elsevier B.V. All rights reserved.