Abstract: The uncertainty of nitrogen oxides in the air by UV spectrophotometry was evaluated according to Evaluation and Expression of Uncertainty in Measurement（JJF 1059.1-2012）.The samples were collected and determined by national standard analysis method.It showed that the relative standard uncertainty in sampling and analysis process were 1.77% and 1.67%,respectively,and the combined relative uncertainty was 2.43%.The main contributors to the sampling uncertainty were the sampling rate of the sampler and the calibration of the sampler flow,the order of uncertainty in the analysis process was standard solution preparation process> sample volume determination> replicated determination of samples> instrument measurements> calibration curve fitting process.The expanded uncertainty of measurement was 0.0024 mg/m³,and the measurement result could be expressed as（0.048±0.002） mg/m³.