The reduction of the energy dissipation induced by defects is essential to achieve the ultrahigh-quality-factor mechanical resonators for the applications of quantum platform and high-sensitivity microelectromechanical (MEMS) and nanoelectromechanical system (NEMS) sensors. Single-crystal diamond (SCD) is the ideal material for high-quality-factor mechanical resonators due to its outstanding mechanical properties and intrinsic low-energy dissipation. To achieve mechanical resonators with extreme properties as well as high reliability, it is desirable to develop all-SCD mechanical resonators. By using a smart-cut method and atomic layer etching to remove the defects within the resonators, we achieve the SCD-on-SCD mechanical resonators with ultrahigh quality factors of over one million at room temperature. The quality factors are one or more orders of magnitude higher than those of the state-of-The-Art MEMS cantilevers based on polycrystalline diamond, single-crystal silicon, and other crystal materials. The diamond MEMS resonators would be highly promising for sensor application as well as for the scheme for coupling with quantum centers in diamond.