Single-crystal anatase TiO2 nanocrystals were assembled into different morphological TiO2 micro/nanoarchitectures, including TiO2 hollow microspheres (HM), solid microspheres (SM), and diamondshaped nanocrystals (NR), by well-controlled solvothermal reactions. The morphology of the resultant TiO2 micro/nanoarchitectures, along with the micro/nanostructural shape, size and crystallinity, can be controlled by varying the concentration of the reactants. It was found that the formation process of TiO2 hollow microspheres might include the hydrolysis of titanium alkoxide with water from the Mannich reaction, the aggregation of TiO2 nanocrystals, the anisotropic crystal growth, and the Ostwald ripening. In addition, we have performed systematic density functional theory (DFT) calculations on the adsorption energy of ethylenediamine molecules on the exposed surfaces and investigated the formation mechanism of single-crystalline nanocrystals. Furthermore, the three TiO2 micro/nanoarchitectures (SM, HM, and NR) were used as photoanode materials in dye-sensitized solar cells (DSSCs) and demonstrated improved light harvesting efficiency and electron collection efficiency, which were much greater than conventional TiO2 nanoparticles under the same conditions. Combined with the large specific surface area and high light scattering ability, the HM-based photoanode achieves the highest power conversion efficiency of 10.66%. Meanwhile, our method provides a versatility for structural engineering of various targeted morphological products to prepare high performance photovoltaic device.