A beta-NMR study of B-12 implanted in diamond was performed in order to investigate the implantation sites and the defects. The maintained polarization of B-12 was measured by use of widely modulated rf around the Larmor frequency (nu = nu (L) +/- 200 kHz) as a function of temperature from 160 K to 320 K. The observed polarization was found to be almost constant at about 0.9 % in this temperature range. The initial polarization for this system was obtained as about 8.1 %. Therefore about 10 % of the implanted B-12 maintained its polarization in this frequency range. Conversely, about 90 % of the implanted B-12 was undetected in the present experiment.