We studied lightly doped Si:P with high-frequency (80-120 GHz) ESR and ESR/NMR double magnetic resonance techniques in the temperature range down to 1.4 K. We found dynamic nuclear polarization of 31P from steady-state ESR measurements with approximately 3.6 T. We derived the nuclear spin relaxation time, T1N, of 31P by analysing the time-evolution of ESR spectra utilizing the dynamic nuclear polarization effect. We derive temperature and magnetic field dependence of T1N and compare with experimental data. Furthermore, from our ESR measurements, we modulate the nuclear polarization of 31P by applying an RF field.