Gold nanoparticles (GNPs) in aqueous dispersion were synthesized on a low-cost glass microfluidic device developed by authors. The effect of a channel width and a flow rate on the size distribution of synthesized GNPs was reported for synthesis of mono-dispersed GNPs. A soda-lime glass substrates was processed by the micropowder blasting. Three holes were processed on the upper substrate, and Y-shaped microchannel was processed on the bottom substrate. Tetrachloroauric (III) acid aqueous solution for Au ion and the mixture of aqueous solution of sodium citrate acid for reducing agent and tannic acid for the protective agent were injected to a microchannel in the device by syringe pump. From the analysis of absorption peak at around 530nm in absorption spectrum, the synthesized GNPs on the device has sharpen peak in comparison with that of GNPs synthesized on the beaker. Moreover, the spectrum with low flow rate showed a sharpened peak in comparison with that of high rate. In the channel width of 260μm, the full width at half maximum (FWHM) at the absorption peak affecting to a distribution of diameter of GNPs were 79.2nm for 0.05mL/min and 92.9nm for 0.06mL/min. Conversely, FWHM in the channel width of 430μm showed almost constant value. From TEM images, the synthesized GNPs using the channel width of 260μm at the flow rate of 0.05mL/min was found to have the mean diameter of 11.5nm and coefficient of variation of 0.09. These results confirmed that the combination of low flow rate and small channel width were attributed to realizing the mono-dispersed GNPs.