Abstract: In an acetate buffer of pH 4.6 and in the presence of CTMAB,ferric ion was quantitatively reduced to ferrous ion and reacted with 5-nitro-1,10-phenanthroline forming an orange red colored complex with its absorption maximum at the wavelength of 502 nm.Molar absorptivity (ε502) of the complex found was 2.48×104L·mol-1·cm-1.Beer′s law was kept in the range from 0.1-2.0 mg·L-1 for vitamin B1.The proposed method was applied to the determination of content of vitamin B1 in its tablets and injections,giving results in comformity with the results found by the method of pharmacopeia.