The present work emphasizes the effect of alkali (NaOH) solution on macromolecular parameters of sisal fiber. The macromolecular parameters of alkali treated sisal fiber were computed along with air-dried sisal fiber by treating fiber as a non-ideal two-phase system characterized by continuous variation of electron density at the phase boundary. Small angle Xray scattering (SAXS) technique was employed to evaluate the macromolecular parameters, using correlation functions, applying the theories of Vonk, Ruland and Misra et al. The finite value of width of transition layer suggests to treat the fiber to be a non-ideal two-phased system. The evaluated macromolecular parameters are: transversal periodicity, specific inner surface, volume fractions of matter and void phases, transversal lengths in both phases, range of inhomogeneity, volume fraction of transition layer and length of coherence.