We report the results of a device performance of a boron-doped diamond metal semiconductor field effect transistor (MESFET) grown on a heteroepitaxial diamond substrate using microwave plasma chemical vapor deposition. The 120 nm-thick lightly boron-doped p-type diamond layer indicates sheet resistance of 6 GΩ/sq. at room temperature (RT). A two-terminal buffer leakage test with pad distance of 10 μm shows that a heteroepitaxial grown diamond substrate withstands more than 3 kV. The fabricated device with gate–drain distance (Lgd) of 10 μm exhibits a drain current density (Id) of 66 nA/mm (gate voltage (Vsg) 0.5 V, drain voltage (Vds) 30 V, and at RT), and at 463 K, the Id increases to 9.4 μA/mm. The device with Lgd of 30 μm shows a high breakdown voltage of −2360 V at RT, and at 463 K, breakdown voltage reduces to −515 V owing to an increase in leakage current. A benchmark on breakdown voltage vs. on-resistance reveals that our breakdown result at RT is the highest value ever reported for a bulk p-doped diamond FETs.