Metal nanocrystals of various shapes and sizes show great potential for applications in biomedicine because of their unique optical, thermal, and electrical properties. However, it is technically challenging to achieve delicate control over reaction conditions to successfully prepare nanocrystals with desired shapes. With the advent of microfluidic technology, sufficient mixing and precise parameter control can be achieved in nanoparticle synthesis to ensure high reproducibility, uniformity, and stability in nanosynthesis. Additionally, a multi-step reaction can be implemented using a microfluidic chip to simplify the synthetic process. Here, we present a microfluidic platform that allows the reaction parameters to be varied in a continuous fashion to screen for conditions that enable the synthesis of gold nanocrystals with a high aspect ratio. By changing the relative flow rates among AgNO3, gold seeds, and L-ascorbic acid (AA), various reactant concentrations were tested for the nanosynthesis. After only several minutes of on-chip nanosynthesis, spherical nanocrystals ranging in size from 13 to 40 nm and nanorods with aspect ratios ranging from 1.4 to 4.3 were prepared with high reproducibility. Compared with conventional off chip batch synthesis, which requires hours to complete, the microfluidic platform offers the advantages of high efficiency, precise parameter control, low reagent consumption, minimal human intervention, and multi-step synthesis. This on-chip synthesis platform is expected to have broad prospects in both chemistry and biomedicine. (C) 2018 Elsevier B.V. All rights reserved.