A rotational excitation spectrum of SO2 [A 1A2(511) ← X 1A1(000)] at about 33331cm-1 in free jets was observed by using the forward degenerate four-wave mixing (DFWM). Twelve lines of the G band and 11 ones of the E band were marked based on the rotational constants according to Hamada's result [Can. J. Phys. 53(1975) 2555]. The relation between the DFWM signal intensity and the pressure of the buffer gas in free-jets was experimentally investigated.