To evaluate the technical of microbial coal desulfurization during the storage in coal dumps, microbial pyrite oxidation in a packed column reactor with Thiobacillus ferrooxidans has been investigated. For microbial desulfurization in a packed reactor system, coal particle size over 1.0 mm with uniform size distribution seems to be most suitable as fas as drainage behavior and accessability of pyrite are concerned. When coal samples of 1∼2 and 2∼4 mm particle size were size were used, about 32∼42% of pyritic sulfur was removed within 70 days. The rate of pyritic sulfur oxidation was in the range of 348∼803 mg S/kg coal ·d, and the sulfur removal rates in packed columns were about 15∼25% of those in suspension cultures. Without any circulation of liquid medium, microbial coal desulfurization could be possible by the inoculation of T. ferrooxidans along on the coal dump. It was concluded that a microbial percolation process is one of possible processes for the desulfurization of high sulfur coal during a long-term storage.