Device size scaling of pseudo-vertical diamond Schottky barrier diodes (SBDs) has been characterized for high-power device applications based on the control of doping concentration and thickness of the p- CVD diamond layer. Decreasing parasitic resistance on the p+ layer utilizing lithography and etching makes possible to get a constant specific on-resistance of less than 20 mOhm-cm2 with increasing device size up to 200 µm. However, the leakage current under low reverse bias conditions is increased markedly. Due to the increase in the leakage current, the reverse operation limit is decreased from 2.4 to 1.3 MV/cm when the device size is increased from 30 to 150 µm. If defects induce an increase in leakage current under the reverse conditions, the density of the defects can be estimated to be 104–105/cm2. This value is 5–10 times larger than the density of dislocations in single crystal diamond substrate.