The principal hygienic problem caused by livestock industry is the odor exposed to farm workers. This study was performed to assess air cleaner efficiency for reducing odor through on-site evaluation. The concentration of ammonia and hydrogen sulfide, which are major odorous compounds generated from livestock building, were monitored by realtime direct recorder. The odor mixture was measured by air dilution method applying human noses of five panels. Their reduction efficiencies were represented by difference between initial concentration exhausted by non-treatment and concentration measured after treatment of respective control mechanism (water, germicide and plasma ion) of air cleaner. Mean levels of ammonia and hydrogen sulfide were 1.84 (SD:0.22) ppm and 76.83 (SD:1.37) ppb for non-treatment, 1.23 (SD:0.09) ppm and 59.07 (SD:2.68) ppb for wet scrubber (water), 1.08 (SD:0.03) ppm and 58.55 (SD:1.62) ppb for wet scrubber (germicide), and 0.96 (SD:0.03) ppm and 53.66 (SD:1.37) ppb for plasma ion, respectively. Mean dilution factors of odor mixture were 100 for non-treatment, 66.9 for wet scrubber (water), 144.2 for wet scrubber (germicide), and 66.94 for plasma ion, respectively. Based on the results obtained from on-site evaluation, ammonia and hydrogen sulfide showed the mean reduction efficiency of 40% and 25.7% compared with non-treatment process of air cleaner, respectively. In the case of odor mixture, the highest dilution factor was observed at wet scrubber (germicide) compared with other control mechanism of air cleaner. Keywords : Air cleaner, Dilution factor, Odor mixture, Pig building