On any industry where the natural fibers are used, chemical composition of the fibers is an important parameter to determine, since it allows to predict the product quality. On the other hand, TAPPI standard developed a well described methodology to determine the chemical composition; however, their methodology is time and resource consuming, and furthermore it requires well trained specialist. An alternative to determine the chemical composition is to use the true density, since the density of the fiber constituents are different. In this study, we present a clear relation between the true density and the three major constituents (cellulose, lignin and extractives) with the fitting parameter R > 90%, for the Stipa obtusa base fibers.