Alkylbenzenes used as the synthetic lubricant base stocks were composed of the mixture of the various kinds of aromatic hydrocarbons. Their compositions have affected on the quality of synthetic alkylbenzene lubricants. Therefore, the rapid and accurate methods for the composit ional analysis are important. In this study, the compositions of the alkylbenzenes (Hv. LAB, FHv. LAB, Hv, BAB, DAB[HF], DAB[$AlCl_3$]) as synthetic base stocks have been investigated according to six average structural parameters(Tar, Nal, Asub, $ar{n}$, nb, $Talpha$) in the view of the molecular structures by $^{13}C-NMR$ spectroscopy. The experimental results of the oxidation $varepsilon$ thermal stability tests have been related to the results of the molecular structural analysis.