An optimization has been performed for the parameters of an Anisotropic United Atoms (AUA) intermolecular potential for thermodynamic property prediction using both Gibbs ensemble and NPT Monte Carlo simulations. The model uses the same parameters as previous AUA models for the aromatic CH, alkyl CH2, and methyl CH3 groups as well as the CH and CH2 groups for olefins. The optimization procedure is based on the minimization of a dimensionless error criterion incorporating various thermodynamic data of p-xylene at 311 and 491 K. The model has been evaluated on a series of alkylbenzenes and styrene including toluene, o- and m-xylene, trimethylbenzene, n-propylbenzene, n-hexylbenzene, and n-dodecylbenzene from ambient temperature to near-critical temperature. Vaporization enthalpy, liquid density, and normal boiling temperature are reproduced with an average error of 2% or lower. Although the proposed AUA model is very simple, as it does not include any electrostatic charges, it accounts fairly well for the influence of the alkyl substituents over a large range of temperature and carbon number.