A new manufacturing process and the characterization of high-aspect-ratio metal microchannel plates for microelectronic cooling applications are reported in this article. A nickel-based microchannel cooling plate, with channels of width 20 μm and aspect ratio up to 3.6:1, has been successfully fabricated using a modified UV-LIGA process. Similar metal microstructures, based on electroplated copper, have also been obtained with a width of 15 μm and an aspect ratio of up to 5:1. In both cases, an over-plate technology was used to electroform the metallic microchannel plates in a single manufacturing step. Hydrodynamic and cooling characteristics of the microchannel plates such as flow rate and heat resistance have been measured. A heat transfer coefficient of 511 W m −2 K −1 for a flow rate of 120 l h −1 has been obtained for the 20 μm wide nickel-based microchannel.