In this work, we performed a detailed theoretical structural and energetic study of the majority triacylglycerols (TAGs) contained in olive oil, their trans isomers and the fatty acids composing them in order to better understand the chromatographic analysis. The geometries of all the studied structures, their relative equilibrium energies and their harmonic frequencies were determined. We carried out energy calculations by DFT method (B3LYP/6- 31+G(d)) for fatty acids. We conducted semi empirical calculations (AM1) for larger triacylglycerol molecules. We showed that trans fatty acids are more stable than cis ones. We also showed that triacylglycerols may exist in two possible shapes. For the TAGs that contain the same number of atoms, we compared the relative equilibrium energies and we showed that when the degree of unsaturation increases, the triacylglycerol becomes less stable. The structural and energetic properties obtained in our work are a first step to understand the HPLC phenomenon because they enable us to determine all the needed theoretical parameters to correlate with the retention time of TAGs in HPLC.