The sweet potato residue being at the initial moisture content 50-58%, initial pH 3.5-4.3, supplemented with rice bran, and minerals, and incubated at 20-26 degrees C for 5 days was the optimal conditions for protease production with Aspergillus niger NTU-AM-1 by solid state fermentation. Protease could be recovered by shaking at room temperature for one hour and extracted with five times volume of 0.1% NaCl solution. The yield of protease was 814 units per gram dry weight of substrate. Partially purified protease with DEAE-Sephacel column chromatography was thermally stable and able to retain 80-100% of activity in pH 4.0-5.5 at 55 degrees C for 40 minutes. In addition, the activity of protease was stimulated by the presence of EDTA and cysteine, but was inhibited by the addition of HgCl2.