¹²C isotopically enriched polycrystalline diamond free-standing plates were synthesized by microwave plasma-assisted chemical vapor deposition. Diamond plates 30 mm in diameter and 100–600 µm in thickness were successfully obtained at a typical growth rate of 3–5 µm/h. The ¹²C isotopic enrichment was 99.997%, whereas the nitrogen content was as low as 4 ppb. The preferential orientation of the <110 > crystallographic direction normal to the substrate plane was proved using the X-ray diffraction pattern. The sharp Raman peak with a low background and a clear free-exciton recombination emission in a cathodoluminescence measurement at room temperature indicate that the polycrystalline diamond has a substantially high crystalline quality.