During the aerobic composting of livestock excrement, or leachate, which is generated in the composting process, is distributed through a bulking agent. In this research, a specific microorganism expected to reduce the emission of odor was activated in livestock excrement supernatant, which is similar to the leachate used for water control. While the microorganism was being diffused, odor properties were examined. The bulking agent, in which the specific microorganism was activated, was produced by applying a bioreactor filled with pumice and pellets to the treatment process of livestock excrement activation sludge method. The dominant microorganisms were Pseudomonas genera. Experiment results showed ammonia content was lowered as to 0.27 ppm 1n 15 days; however, it remained at levels of 3.15 ppm in the control reactor. After the composting time was completed, the ammonia concentrations were 0.07 ppm and 3.43 ppm, respectively. The complex odor in the test reactor was 28~30 times greater than of dilution, but was 42~45 times greater in the control reactor. Hydrogen sulfide and methyl mercaptan were not detected in either reactors. Ammonia was thought to be the major odor-producing source during the aerobic composting process of livestock excrement, and it was expected that the odor could be reduced by spraying a microorganism -activated solution as a bulking agent. Keywords : Microorganism activated solution, Livestock, Odor emission, Compost, Complex odor