The predissociation mechanism of the C state of SO₂ have been investigated by the measurements of degenerate four-wave mixing (DFWM), laser-induced fluoresence (LIF) and photofragment yield spectra. he DFWM spectrum of jet-cooledSO₂ was measured under a high power condition where the DFWM signal corresponds well to the absorption spectrum. Relative flrorescence quantum yields of various vibronic bands were obtained by comparing the DFWM spectrum with the LIF spectrum. It was found that the fluorescence quantum yields of the vibronic levels involving anti-symmetirc stretching (ν₃) vibration are sonsiderably smaller than those of other levels. The result indicates that the nonradiative process of the C state is initiated as the internal conversion o the X state and ν₃ acts as the promoting mode. By observing the O atom photofragment yield spectrum we confirmed that the predissociation occurs from the (1,4,2) level. Vibronic dependence of the rise time of the O atom generateion was found to be in accordance with those of the fluorescence decay rate. The obtained results represent that the dissociation occurs immediately after the internal conversion to the X state.