The 9 October 2006 underground nuclear explosion (UNE) test in North Korea was well monitored by dense regional seismic stations in South Korea, Japan, and China. This observation allows extensive investigation of the regional source properties of the UNE. The moment for isotropically radiated energy from the UNE is estimated to be 2.92×1014 N m. Source spectra of major regional phases from the UNE are studied by inverting for apparent moments, corner frequencies, overshoot parameters, attenuation factors, and frequency power-dependence parameters. The overshoot parameters of P phases from the UNE are estimated to be high, while those of S phases are estimated to be significantly low. The inverted source spectra agree well with conventional models. The low overshoot parameters of S phases suggest that their excitation sources may be different from those for Pn and Pg. It is shown that Pn/Lg and Pg/Lg amplitude ratios are useful for discriminating between UNEs and natural earthquakes in the frequencies of 1–8 Hz.