The electrochemical oxidation of ammonia was done at electrochemically Pt-modified microcrystalline and polycrystalline boron-doped diamond (BDD) thin films from sp3 Diamond Technologies and Element Six. The Pt electrodepositions were done by chronoamperometry and cyclic voltammetry using a 1 mM K2PtCl6 solution in 0.5 M H2SO4. The Pt-BDD electrodes were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction, and cyclic voltammetry. The Pt-BDD electrodes were compared by SEM and ammonia oxidation. At BDD electrodes, cyclic voltammetry depositions favors the formation of cubic and cauliflower Pt nanostructures at microcrystalline and polycrystalline BDD electrodes, respectively. On the other hand, chronoamperometry electrodeposition favors dendrite formation structures in both BDD electrodes. Platinum electrodeposited by chronoamperometry favors the formation of platinum (100) facets, which enhances ammonia oxidation. Pt-microcrystalline BDD showed the highest ammonia oxidation peak current densities when compared to Pt-polycrystalline BDD.