It was observed that levansucrase from Bacillus natto became unstable and was easily inactivated when the salts were removed from the enzyme solution, while the enzyme was stable for long time in a buffered saline. After modification with periodate oxidized yeast glucomannan, the enzyme increased thermal stability up to 45degreesC, in which it conserved more than 90% of its activity after 15 min treatement. The optimum temperature was also shifted from 40degreesC in the case of original enzyme to 50degreesC for the modified enzyme after 10 min reaction time. The half-life time increased significantly from 9 min to 55 min at 50degreesC, however it increased from 30 min and 22 min respectively at 40degreesC and 45degreesC to more than 1 h at the same temperature. The content of carbohydrates of modified enzyme was 25% that increases the molecular weight froth 57 KDa to 80 KDa. The products from sucrose by the modified enzyme were the same as the case using original enzyme. Namely, the products confirmed were levan and 3 kestoses (6-, 1-, and neo-kestose).