We present first principle investigation of the influence of platinum nanoparticle shape and size on the oxygen reduction reaction activity. We compare the activities of nanoparticles with specific shapes (tetrahe- dron, octahedron, cube and truncated octahedron) with that of equilibrium particle shape at 0.9 V. Furthermore, the influence of support is assessed by looking at the particles with and without support interactions. The equilibrium shape is determined by calculating the changes in surface energies with potential for low-index platinum facets; (111), (100) and (110). This has been done by explicitly taking the coverage of oxygenated species into account. A kinetic model derived from counting the number of sites shows that the theoretical activity obtained for equilibrium particle fits well with experimental data. Particles with *3 nm diameter are found to possess the highest activity.