This work deals the effect of acetylated and non-acetylated sisal fiber on thermal degradation of polypropylene. Applying the R-G method at constant conversion levels of 0.1, 0.3, 0.5, 0.7 and 0.9 to thermograms of the "PP/untreated sisal fiber" blend, E-a values of 99, 213, 224, 187, and 145 kJ/mol were obtained, whereas they were 99, 299, 255, 205, 154 kJ/mol for the "PP/treated sisal fiber" blend. Oil the other hand, with the R-G method at constant temperature, activation energies within the range of 156-417 kJ/mol were obtained for the "PP/treated sisal fiber" blend and within the range of 126344 kJ/mol for the "PP/untreated sisal fiber" blend. Additionally, the method establishes as dominant the following decomposition mechanisms: three-dimension limiting surface reaction between both phases, diffusion in two and three dimensions and first-order random nucleation and nuclei growth.