Diamond has an attractive interest as one of next-generation power electronics materials. Recently, development of thermally-stable, high-current diamond field effect transistors (FETs) with Al2O3 gate and passivation dielectrics deposited by an atomic layer deposition (ALD) technique were reported to be as large as 1 A/mm by Waseda Univ. and NTT groups. In addition, since the sheet hole density in the hydrogenated-diamond surface was reported to be as high as 1E14 cm-2 which was one or two orders larger than other semiconductors. Therefore, we should use such the big advantage and then have to develop the high-k gate dielectric for diamond in order to control the high-density hole carrier. Since the high-k dielectric provides the large capacitance at a given gate voltage, the controllable carrier density is predicted to be increased with increasing the dielectric constant. For this purpose, as a first step to search the best high-k insulator material to the diamond, we have demonstrated the diamond FETs with high-k HfO2/HfO2, LaAlO3/Al2O3 Ta2O5/Al2O3, and ZrO2/Al2O3 stack gates prepared by a combination of sputter-deposition (SD) and ALD techniques. Since the FET property is sensitive to the interfacial states between the diamond and dielectric and the border traps in the dielectrics, it is essential to obtain the guideline for developing the excellent gate dielectric for diamond. In this paper, we will show why the high-k dielectric insulator is required for diamond and demonstrate the LaAlO3, HfO2, Ta2O5, and ZrO2 as the gate insulator of the FETs using the hydrogenated diamond (H-diamond) p-type channel. In addition, to understand the interface property between the diamond and dielectric, we will investigate the electric properties of interface between the H-diamond and insulator.