Polymorphism of trielaidin C3H5(OCOC17H33)(3), which is a typical monoacid trans-unsaturated TAG, has been studied with X-ray diffraction and vibrational (IR and Raman) spectroscopy. It is clarified that the beta phase changes the conformation around trans-C = C bond depending on its crystallization conditions. As for the internal rotation angle of C-C bonds next to trans-C = C bond, the beta phase grown from a hexane solution shows a significant deviation from the standard skew or skew' conformation, which was found in the beta phase grown from a methanol-chloroform-toluene mixed solution. Contrary to the ordered molecular structure of the beta phase, the alpha phase grown from a melt at high supercooling has a disordered structure at the glycerol backbone. The polymethylene chain between trans-C = C bond and ester linkage is also conformationally disordered in the alpha phase. (C) 2002 Elsevier Science B.V. All rights reserved.