The availability of natural fibers with their high performance and low cost contributes to a healthy ecosystem and fulfills the economic interests of the society. Sisal fiber, which is easily cultivated, is one of the most widely used natural fibers. For the present studies, characterisation of locally available sisal fiber of M.P. state in India was carried out with the investigation on the effect of alkali treatment with a wide range of different concentrations on the mechanical properties of sisal fiber. The physical and chemical properties of sisal fiber were analyzed with known methods and results were found to be similar to earlier reported values. Alkali treatment was carried out with different concentrations and mechanical properties were evaluated with Lloyd Fiber Testing Instrument. The results show that the chemical treatment enhances the mechanical properties of fiber up to a limit and is found to decrease with further increase in concentration. Scanning Electron Microscopy was used to investigate the morphology of untreated and alkali treated sisal fibers which indicated that the different concentrations of alkali alter the fiber surfaces with variations. Sisal fibers before and after alkali treatment were characterised further by thermogravimetry to establish their thermal stability, and measurements showed that alkali treated fiber became more thermal resistant than the untreated fiber and it was also noticed that thermal resistance decreases at higher concentration. (C) Koninklijke Brill NV, Leiden, 2011