The acaricidal effects of the active constituent isolated from Ostericum praeteritum oil and its derivatives were compared with that of benzyl benzoate using fumigant and contact toxicity bioassays against Tyrophagus putrescentiae mites. The active compound of O. praeteritum was isolated by various chromatographic techniques and was identified as 3-methylphenol (C7H8O). On the basis of 50% lethal dose values, 3-methylphenol (1.42 μg/cm2) was determined to be 9.18 times more effective than benzyl benzoate (13.04 μg/cm2), followed by 6-fluoro-3-methylphenol (2.18 μg/cm2), 4-isopropyl-3-methylphenol (2.53 μg/cm2), and 6-chloro-3-methylphenol (4.03 μg/cm2), against Tyrophagus putrescentiae mites in the fumigant bioassay. In the contact toxicity bioassay, 3-methylphenol (1.03 μg/cm2) was the most-toxic compound against T. putrescentiae mites, followed by 6-fluoro-3-methylphenol (2.09 μg/cm2), 4-isopropyl-3-methylphenol (2.11 μg/cm2), 6-chloro-3-methylphenol (3.78 μg/cm2), and benzyl benzoate (10.33 μg/cm2). These results indicate that the introduction of chloro, isopropyl, and fluoro functional groups to the 3-methylphenol skeleton increased the acaricidal activity. Therefore, 3-methylphenol and its derivatives could potentially be used as natural acaricides against T. purescentiae.