The substrate selection for the diamond growth is important. A single crystal of potassium tantalum niobate was chosen to deposit diamonds by microwave plasma chemical vapor deposition (MPCVD) techniques. The surface morphologies and microstructures of the as-deposited diamonds were characterized by SEM, XRD and Raman spectroscopy. The photocatalytic properties were studied and all samples show good ability to degrade Rhodamine B. The degradation rate increases with the growth time of polycrystalline diamond samples. This research provides a novel substrate for growing polycrystalline diamond, and it has made the exploration of diamond applications in photocatalysis.