Abstract: By optical fiber photospectrometer, trace nitrogen in uranium and uranium-niobium alloy was determined with thymol as a chromogenic reagent. In the presence of sodium hypochlorite solution whose effective chlorine content was 0.1%-0.14%, ammonia was reacted with thymol to form a blue indophenol compound with maximum adsorption wavelength at 660nm. The determination limit of this method is 20μg/g. The relative standard deviation is less than 15%. The recovery is 88%-106%. These results are consistent well with that obtained with a traditional spectrophotometer.