A durable composite diamond electrode is disclosed which comprise at least a relatively thicker conductive UNCD (Ultrananocrystalline Diamond) layer, with low deposition cost, on a substrate underlying a relatively thinner conductive MCD (Microcrystalline Diamond) layer. The electrode exhibits long life and superior delamination resistance under extremely stressed electrochemical oxidation conditions. It is hypothesized that this improvement in electrode reliability is due to a combination of stress relief by the composite film with the slightly “softer” underlying UNCD “root” layer and the electrochemically durable overlying MCD “shield” layer, an effective disruption mechanism of the fracture propagation between the compositing layers, and thermal expansion coefficient match between the diamond layers and the substrate. The diamond composite electrode can be applied to any electrochemical application requiring extreme voltages/current densities, extreme reliability or biomedical inertness such as electrochemical systems to generate ozone, hydroxyl radicals, or biomedical electrode applications.