A flow injection (FI) kinetic spectrofluorimetric method is described for the determination of osmium (IV) and the possible mechanism of catalytic reaction is discussed. The method is based on the fluorescence enhancing reaction of o-vanillin furfuralhydrazone (OVFH) with potassium bromate, which is catalyzed by Os (IV) in water medium at pH 6.10 and 45℃. OVFH is newly synthesized and its ionization, IR and elemental analysis are established. Under these experimental conditions, the oxidized product of OVFH has excitation and emission maxima at 337 and 490 nm, respectively. The linear range of this method is 0-600ng/ml−1 with the R.S.D. of 1.2%. The detection limit is 1.0ng ml−1 of Os (IV). A high analysis rate of 24 samples h−1 is obtained by the FI method. The proposed method is applied successfully to determine Os (IV) in synthetic mixture and mineral samples, and the results are well consistent with the standard values.