Noise equivalent temperature difference is of great importance to the evaluation of the performance of infrared systems. It is helpful to the performance improvement, cost reduction and production periode shortening of infrared systems. Noise equivalent temperature difference is one of the important specifications for measuring the performance of infrared detector systems. It is related to the total atmospheric transmittance and detector performance parameters etc.. The expression of the noise equivalent temperature difference is deduced. The performance of a particular infrared detector is characterized by using the noise equivalent temperature difference and various performance influence factors are examined through a special program we wrote.