The potential application of time domain EPR spectroscopy to an organic molecular spin system was examined, particularly focusing upon quantum spin transient nutation phenomena. A nitronyl nitroxide-based biradical with an excited triplet state (S=1), p-phenylenebis(alpha-nitronylnitroxide), was adopted as a simple model system for studying the nutation spectroscopy which enables us to discriminate between S greater than or equal to 1. and S=1/2 states. In the low temperature region below 10 K, an S=1 spin of the thermally excited biradical molecule and an S=1/2 spin of an impurity molecule were found to coexist in the crystalline solid state of p-phenylenebis(alpha-nitronylnitroxide). Field-swept echo-detected EPR spectra reproduced the cw spectra of the biradical powder, Echo-detected nutation spectra exhibited two distinct maxima of the echo intensity at the nutation frequencies expected for S=1 and S=1/2 spins, indicating that the biradical solid was identified to be a mixture of the S=1 and S=1/2 species as expected from the static susceptibility and crystal structure analysis of the biradical. The nutation method based on FT pulsed EPR spectroscopy was demonstrated to be complementary to conventional cw EPR and susceptibility measurements.