The present invention relates to a plant mangrove-associated fungus Curvularia lunata, and a simple and efficient method of obtaining high yield of pure mannitol from a plant mangrove-associated fungus Curvularia lunata, said method comprising steps of cutting the leaves into small pieces and placing it on potato dextrose agar (PDA) plates for 48 hours, maintaining the culture at temperature ranging between 26-47.degree. C. and salinity ranging between 4-32 ppt with occasional stress conditions for about 17-19 days to obtain mycelial mat, sonicating the mat to lyse the cells, extracting crude from the sonicated mat using methanol repeatedly, concentrating the crude, treating the concentrated crude extract with solvents of increasing polarity, obtaining aqueous fraction as white powder residue after the treatment containing crude mannitol, purifying the crude mannitol by chromatography to obtain pure mannitol with about 75% of the total crude extract.