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 P-31 from steady-state ESR measurements with approximately 3.6 T. We derived the nuclear spin relaxation time, T-1N, of P-31 by analysing the time-evolution of ESR spectra utilizing the dynamic nuclear polarization effect. We derive temperature and magnetic field dependence of T-1N and compare with experimental data. Furthermore, from our ESR measurements, we modulate the nuclear polarization of P-31 by applying an RF field.