In this work, we describe a solution-based synthesis of monodisperse Cu2O nanocrystals with controllable sizes in the nanoscale regime. Two types of nanocrystals, cubes and rhombic dodecahedra unifaceted with either {001} or {110} crystal planes, have been prepared at a 100% morphological yield. In particular, synthetic parameters and formation processes of the Cu2O nanocrystals have been investigated in detail, and a range of well-oriented supercrystals/superlattices built from the two types of nanobuilding blocks have been attained for the first time. It has been revealed that n-hexadecylamine used in the present work plays multiple roles: it serves as a chelating ligand to form [Cu(NH2C16H33)(4)](2+) complex precursor, as a phase-transferring agent to transfer divalent Cu2+ ions into the organic phase, as a reducing agent to generate monovalent Cu+ (i.e., Cu2O), as a passivating adsorbate to control crystal morphology, and as a surface capping agent to generate self-assemblies of nanocrystals via van der Waals interaction. Apart from synthesis and self-assembly, disassembly and reassembly of Cu2O nanocrystals have also been investigated. The disassembly processes are accompanied with aggregative growths of nanocrystals, which can be attributed to a combined process of "oriented attachment' and Ostwald ripening, leading to permanent engagement and enlargement of nanocrystals. Finally, our self-assembled nanocrystals of Cu2O show a lower detection limit, lower operating temperature, and higher sensitivity in ethanol vapor detection, compared with other Cu2O-based alcohol sensors reported in the recent literature. A greater depletion layer of carrier and a relatively small contact potential may account for the observed sensing enhancement in the sensors made from the organized Cu2O nanocrystals.