We demonstrate the on-chip synthesis of Au nanoparticles achieved by a microwave-induced reaction in a microfluidic channel. The chip structure consists of a waveguide and a microchannel through the inside of the waveguide. A post-wall waveguide, which configures the metallic post on the side wall continuously, is utilized to confine microwave fields. It allows to insert a solution inside the waveguide by microchannel passing between the metallic posts. In order to realize a miniaturization of the chip, the 24.125 GHz industrial, scientific, and medical (ISM) band is utilized instead of the commonly used 2.45 GHz ISM band. When pure water is heated in the microchannel under the microwave input power of 3.0 W, it has been confirmed that the temperature increases to 70℃. In this paper, this chip is adopted to synthesis Au nanoparticles via microwave-induced reaction. After the irradiation of the microwave power 3.0 W and the irradiation time 5 min, we have observed absorbance and dynamic light scattering of the reactant, and the results indicated generation of Au nanoparticles. This microwave synthesis system allows us to achieve automatic and fast selective synthesis of nanoparticles in the solution.