Herein, a low-cost 3D printed photo-reactor system for the photo-chemical synthesis of silver nanoparticles with different morphologies is presented. Silver colloids with a variety of colors were obtained after light irradiation in continuous flow. The combination of milli-fluidic devices and LED light illumination clearly speeds up the growing process (hours) compared to batch systems (day to months) with similar initial colloids. Moreover, the effect of residence time, and reactant concentrations were investigated. Results indicated three stages, such as induction, growth, and termination during photo-aggregation within the continuous flow reactor. Moreover, H2O2 treated samples grew 3.5 times faster than non-treated AgNP colloids, with a complete NP formation under 4 h.