Abstract: A method to evaluate the sub-pixel size and temperature of fire spot was presented by using the multiple infrared channel data from polar-orbiting meteorological satellite sensor under various fire spot conditions with the different infrared band data, for example, the mid-band (3.7μm) and thermal-band (11μm), or short infrared band (1.6 μm) and thermal-band (11μm). The Newton's iteration method can be used to find an accurate solution from a dual non-linear coupled equation established by the expression of mixed pixel. For the different channels in the same infrared band, the relationship, between sub-pixel size and temperature of active fire vs the brightness temperature increment of mixed pixel in 11μm channel and difference of brightness temperature increment of mixed pixel in 11μm and 12μm channel, can be established by generating a look up table, and from the table, the evaluating of sub-pixel size and temperature of fire spot can be derived from the brightness temperature of a mixed pixel.