Using an activated carbon felt (ACF, KF-1600), the applicability of an ACF passive sampler in monitoring organic vapors was evaluated. The evaluated results of the ACF passive sampler were compared with those of a 3 M (Model 3500) passive sampler. At low-humidity condition (8 +/- 3% RH), the sampling rates of the ACF passive sampler for volatile organic vapors were within the range of +/-25% from the results of a charcoal tube, which was the reference method recommended by NIOSH. However, at high humidity condition (90 +/- 5% RH), even though the sampling rates for toluene, MIBK, and PCE were within the range of +/-25%, the sampling rates of ACF sampler for n-hexane and MEK decreased steeply with time after being high at the beginning of the exposure time. Under this condition, the sampling rate of the 3 M passive sampler for MEK was also high at the beginning of the exposure time. In case of the ACF passive sampler, the sampling rates of the relatively strong adsorbates such as toluene, MIBK, and PCE were much less affected by humidity than those of n-hexane and MEK which were weak adsorbates.