Nanoparticle self-assembled monolayers (Nano-SAMs) have drawn great attention for their unique applications in many fields such as plasma, surface-enhanced optics, sensors and catalysts. However, how to fabricate high-coverage and densely-packed Nano-SAMs in a limited time is still a challenge, especially for the ultra-thin Nano-SAMs (< 10 nm). In this paper, high-coverage and densely-packed ultrathin Nano-SAMs were fabricated by using ultra-small nanoparticles (e.g. nanoclusters and quantum dots) to deposit onto substrates in organic/water mixture via the continuous dropping of organic solvent. Compared with the conventional methods such as the electrophoresis deposition and the soaking method, nanoparticles could formed higher coverage and more densely-packed Nano-SAMs with few defects through a shorter period of time by using this mixed-solvent precipitation method. This mixed-solvent precipitation presents a facile approach to design and construct high-coverage densely-packed Nano-SAMs.