According to the previous experimental results, the band emittance of two materials were computed for 8-14 mm bandwidth in infrared measuring. The band emittance of several materials was surveyed by a simple experiment. The experiment and reckoning show that there is some kind of functional relation between band emittance and temperature. If the object measured is non-gray, and emissivity is regarded as a constant, acute measurement error will be generated for band pass radiation thermometer and thermal imaging system. The band emittance is nearly linear with the temperature for nonmetal and metal in vacuum by primary analysis. The fitted function equation can be used as the modification of band pass radiation thermometer and thermal imaging system, and the band emittance not only simplifies the calculation, but also improves the accuracy of measurement.