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Penicillium digitatum (Pers.:Fr) Sacc. is the major cause of green mold disease in orange trees grown for orange production industry in Thailand. In this study, antagonistic microorganisms were isolated from various parts of healthy orange trees. A total of 121 microbial isolates were recovered from which 15 isolates showed inhibitory effect against P. digitatum by spot test technique. The inhibitory effect of these 15 isolates was also tested by agar disc diffusion method using culture filtrate from nutrient broth. It was found that isolate W1L1, D1L2 and W1L9 gave the best inhibition with a clear zone of 21, 15 and 14 mm in diameter, respectively. The effect of culture filtrate on spore germination was also investigated. The result showed that isolate W1L1 gave the highest inhibition after 8 hours of incubation with the lowest germination rate of 2.4%. Morphological and biochemical characterizations revealed that these isolates were members of the genus Bacillus and identified as B. pumilus.