Unexpected high DNP enhancements of more than 10 have been achieved in liquid water samples at room temperature and magnetic fields of 9.2 T (corresponding to 400 MHz H-1 NMR frequency and 260 GHz EPR frequency). The liquid samples were polarized in situ using a double-resonance structure, which allows simultaneous excitation of NMR and EPR transitions and achieves significant DNP enhancements at very low incident microwave power of only 45 mW. These results demonstrate the first important step toward the application of DNP to high-resolution NMR, increasing the sensitivity on biomolecules with small sample volumes and at physiologically tow concentrations.