A transient absorption detected magnetic resonance technique is used to explore the electron spin nutation in a polymethylene-linked biradical system under a strong electromagnetic field (B-1). The application of pulsed microwave with the duration successively incremented to the EPR spectral center shows a double frequency oscillation of the absorbance signal along with the principal single one due to one electron spin nutation (omega (1) = g mu (B1)/h). The former signal is explained by simultaneous strong microwave irradiation to two spins in the biradical. This is the first observation of a 2(omega1) spin-nutation signal by monitoring the chemical reaction change of a transient biradical.