Hierarchically assembled tungsten oxide (hydrates) nano/microstructures were synthesized by a crystal-seed-assisted hydrothermal process. Crystal seeds were prepared by refluxing the sol formed by oxidation reaction between W and H2O2. NaCl was used as the capping agent. The assembled nano/microstructures could be controlled to form multiple morphologies: double-layer nano/microbundles, quasi-nanorod arrays, nanorod-spheres, double-layer nano/microdisks and double-layer six-pod prisms. Their morphologies and phase compositions are dependent on crystal seeds, substrate, and precursor concentration. Hierarchical assembly mechanisms are proposed for the formation of the assembled nano/microstructures. The assembled double-layer nano/microstructures are composed of two different phases of tungsten oxide (hydrates), which represent new cases for two-phase nano/microstructures.