This communication reports a spectrophotometric method for the determination of microgram amounts of niobium. 2-(5'-Bromopyridyl-2'-azo)-5-diethylamino-phenol is used as a complexing colorimetrio reagent. It reacts with niobium in H₂O-EtOH medium and in the presence of tartaric acid at 0.02-0.14 N HCl to form a purple-blue ternary complex having an absorption maximum at 626 nm and molar absorptivity of 6.6×10⁴·mole^-1·cm^-1, Boer's law is obeyed in the concentration range of 0~25μg Nb/25 ml. The effects of 40 ions have beon studied and found that only large amounts of Fe³⁺ and Cu²⁺ interfere. However, their interferences can be eliminated by addition of ascorbic acid and thiourea respectively. The composition of the ternary complex of niobium with 2-(5'-brompyridyl-2'-azo)-6-diethyl-aminopenol and tartaric acid has been determined by Job's continuous variation and slope ratio method ag well as the displacement of equilibrium method indicating that niobium may react with 2-(5'-bromopyridyl-2'-azo)-5-diethylaminophenol and tartaric acid in a molar ratio of 1:1:1.The proposed method is simple and rapid with high sensitivity and good selectivity. It.has been applied to the direct determination of niobium in alloy steels without preliminary separation and the results of the determination were reasonably accurate.