Research on antagonistic potential of Gram-positive Bacillus siamensis LDR against two contaminant mold isolates of Aspergillus niger ART and ABP has been done. Antagonistic assay was performed by dual culture method using dual disc culture and pour plate culture. The result test in dual disc culture, based on colony diameter, showed that B. siamensis LDR was able to inhibit Asp. niger ART up to 44.83 % and ABP 39.95 %. In pour plate culture, growth inhibition by B. siamensis reduced the colony diameter of Asp. niger ART up to 87.98 % and ABP 92.43 %. Antifungal fermentation of B. siamensis was carried out in Potato Dextrose Broth using still culture method for 10, 12 and 14 days. Filtrate of fermentation was used as a solvent for PDA and PDB media which were used for antibiosis assays. Antibiosis test showed that the most effective results were obtained from filtrate of 14 days fermentation. Based on colony diameter of Asp. niger ART on PDA showed that there was inhibition of growth up to 74.53 % and 62.13 % for Asp. niger ABP. Growth inhibition in PDB reduced the biomass production of Asp. niger ART up to 79.80 % and 52.38 % for Asp. niger ABP, compared to control. Antibiosis test was also performed using crude extract of filtrate fermentation medium with concentration of 2.500, 5.000, 10.000, and 20.000 ppm. Extraction was done by liquid-liquid extraction method using ethyl acetate as a solvent. Result of antibiosis test using disc diffusion method showed that inhibition of mycelium growth from Asp. niger ART and ABP were already observed at 2.500 ppm.