Nanoparticles present unique physical and chemical properties compared with their counterparts, leading to potential applications in catalysis, electronics, and drug delivery. Since the properties of nanoparticles greatly depend on the size, it is important to synthesize nanoparticles with controlled size and a narrow size distribution. However, the lack of large-scale production methods of nanoparticles with controlled sizes in the absence of organic capping ligands which may interfere with final applications, hinders the potential applications. The thesis focuses on the syntheses of gold nanoparticles (Au NPs) and perovskite nanocrystals with controlled sizes in a continuous manner, combining kinetics study and reaction engineering approaches as well as computational fluid dynamics (CFD). Firstly, the mechanism of the Turkevich protocol, the recommended method for the synthesis of Au NPs by the National Institute of Standards and Technology (NIST), is investigated in batch. The results challenge the traditional belief that the mechanism consists of a reduction reaction and a consecutive disproportionation reaction, revealing that two reduction steps exist and the second reduction is the rate-limiting step. By optimizing the second reduction step, the size of Au NPs can be tuned from 21 ± 3.4 nm to 5.2 ± 1.7 nm. Then, a continuous reaction system for the synthesis of Au NPs between 2-7 nm in the absence of organic steric capping ligands, is developed, with sodium borohydride as a strong reducing agent. It is found that the size of Au NPs can be adjusted between 7.3  2.2 nm and 2.1  0.5 nm by controlling mixing efficiency and speciation of gold precursor. In order to understand the effect of mixing on the size of Au NPs, a novel CFD method is established based on a published algorithm to simulate the accurate concentration distribution, avoiding numerical diffusion (false diffusion), which arises from truncation errors associated with representing the flow equations in discrete form. It is discovered that a higher specific contact area between two streams in mixers, namely mixing potential, leads to more generated nuclei on the interface and smaller Au NPs. The flow synthesis of perovskite nanocrystals with sharp blue emissions is developed in a collaboration. It is shown that the mixing efficiency in the reactor dominates the optical properties of perovskite nanocrystals, by controlling the size of perovskite nanocrystals. The reactor geometry has been optimized to enhance mixing. The photoluminescence of perovskite nanocrystals can be adjusted from peak location 507.0 nm, FWHM (full width at half maximum) 34.9 nm to peak location nm 475.7 nm, FWHM 26.6 nm, to meet the demand of perovskite nanocrystals with sharp blue emissions driven by potential high-quality display applications, by controlling hydrodynamics and mixing efficiency in microreactors and micromixers. In the end, the mixing efficiencies in reactors and mixers are systematically investigated by the novel CFD method. The mixing efficiencies in T- and Y-shaped mixers, as well as cyclone mixers, are analyzed. The mixing efficiency in helical reactors is optimized. Two shortcuts, corresponding to inefficient mixing zones and shortest residence time, are found in the helical reactor. Then, the rational design of alternating axis reactor is proposed to break the two shortcuts and enhance mixing. High efficient mixing systems are proposed by combining cyclone mixers and curved reactors, based on the new understandings of mixing efficiencies in reactors and mixers. The scaling-up effect of single-phase mixing efficiency is systematically quantified. The thesis contributes to the flow synthesis of Au NPs and perovskite nanocrystals with tunable sizes and optical properties to enable their potential applications in catalysis and electronics. In addition, the rationally designed reactors and mixers with high mixing efficiency can be widely applied in the flow chemistry field.