Hierarchical Fe2O3@WO3 nanocomposites with ultrahigh specific areas, consisting of Fe2O3 nanoparticles (NPs) and single-crystal WO3 nanoplates, were synthesized via a microwave-heating (MH) in-situ growth process. WO3 nanoplates were derived by an intercalation and topochemical-conversion route, and the Fe2O3 NPs were in-situ grown on the WO3 surfaces via a heterogamous nucleation. The water-bath-heating (WH) process was also developed to synthesize Fe2O3@WO3 nanocomposite for comparison purposes. The techniques of X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the samples obtained. The results show that ą-Fe2O3 NPs with a size range of 5-10 nm are uniformly, tightly anchored on the surfaces of WO3 nanoplates in the Fe2O3@WO3 samples obtained via the MH process, whereas the ą-Fe2O3 NPs are not uniform in particle-sizes and spatial distribution in the Fe2O3@WO3 samples obtained via the WH process. The BET surface area of the 5wt.%Fe2O3@WO3 sample derived by the MH process is as high as 1207 m2 g-1, 5.9 times higher than that (203 m2 g-1) of the corresponding WO3 nanoplates. The dramatic enhancement in the specific surface area of the Fe2O3@WO3 samples should be attributed to the hierarchical microstructure, which makes the internal surfaces or interfaces in aggregated polycrystals fully be outside surfaces via a house-of-cards configuration, where the single-layered and disconnected Fe2O3 NPs are tightly anchored on the surfaces of the WO3 nanoplates. The gas-sensing properties of the Fe2O3@WO3 sensors were investigated. The gas-sensors based on the Fe2O3@WO3 obtained via the MH process show high response and selectivity to H2S at low operating temperatures. The 5%Fe2O3@WO3 sample shows the highest H2S-sensing response at 150 oC. Its response to 10-ppm H2S is as high as 192, 4 times higher than that of the WO3-nanoplate sensor. The improvement in gas-sensing performance of the Fe2O3@WO3 nanocomposites can be attributed to the synergistic effect in compositions and the hierarchical microstructures with ultrahigh specific surface areas.