In this letter, we investigated random lasing phenomenon in ZnO powder using the pump_probe technique. Under 267nm laser pumping, supernarrow single_mode emission and multi_mode emission were observed by precisely controlling the pumping intensity and the excitation beam size on the sample. The supernarrow emission peaks are much narrower than the normal luminescence and ASE spectra. The time_resolved pump_probe measurement shows that the lifetime of the emission state above the lasing threshold is only a few picoseconds which demonstrates that the formation of supernarrow random lasing bandwidth is due to stimulated emission.