 This paper examined the relationship between cellulose crystallinity, extractive content, and the physical properties of 10 different types of natural fibers. It was found that higher levels of extractives are associated with lower crystallinity and smaller crystallites, which leads to faster degradation and reduced thermal stability. Additionally, it was observed that the thermal decomposition of natural fibers shifts to higher temperatures as their crystalline structure increases. This suggests that the crystalline structure of natural fibers plays a role in determining their thermal stability. Furthermore, the data collected from these experiments can provide valuable insight into the structure and properties of natural fibers prior to their use in composite formulations. This article was authored by Mateus Paletto, Ator L, and Adamir Jay Zadra.