In this study, the effects of the monotherpenic phenol concentration on the properties of biocomposites containing plasticized poly(lactic acid) (PLA) with acetyl tributyl citrate (ATBC) were investigated. The monotherpenic phenols carvacrol (C) and thymol (T) were added to PLA by a melt-blending method. The prepared samples were characterized by means of tensile testing, Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy (SEM), and antibacterial activity tests. The addition of ATBC to PLA resulted in hydrogen bonding between ATBC and PLA. We observed that ATBC, C, and T reduced the glass-transition temperature of PLA. The presence of C and T decreased the maximum degradation temperature slightly. Because of the plasticization effect of the additives, the tensile strength and Young's modulus of PLA decreased, whereas the extent of elongation they experienced before failure increased. This effect was also observed with SEM analysis in terms of plastic deformation at break. The antibacterial activity tests showed that samples containing high concentrations of C demonstrated an improved antibacterial activity against Staphylococcus aureus, Salmonella typhimurium, and Listeria monocytogenes bacteria. We observed that C exhibited a higher inhibition against bacterial strains than T. (C) 2017 Wiley Periodicals, Inc.