Wildfires plays a fundamental intervention in global biogeochemical cycle, by the chemical reaction occurring in the combustion process, and the organic compounds present in vegetation returns to the atmosphere and soil in a cyclical behavior. Therefore, the main objective of this work is to develop a method to estimate the instantaneous fire size in Brazil using Thematic Mapper (TM) aboard of Landsat 5 and Enhanced Thematic Mapper Plus (ETM+) aboard of Landsat 7. The results indicate that active fire to pastures/grasslands is approximately 38% higher than that found for forest areas, 31% higher than the same coefficient used to estimate the fire size in areas of herbaceous and shrub vegetation and 11% higher than the coefficient used in agricultural areas.