The whisker of polyoxymethylene produced in the cationic polymerization process of trioxane is a single crystal of extended chain crystal morphology. Since it has a quite small size of several microns in radius and several tens of microns in length, the crystal structure analysis of POM whisker had been tried by using an electron diffraction technique, but it was easily damaged by an electron beam. We have succeeded in taking the electron diffraction patterns of the POM whisker from the different directions by reducing the electron beam intensity as much as possible and by using a highly sensitive imaging plate detector. Application of the direct method, which is useful for solving the so-called phase problem in structure analysis, allowed us to obtain the helical conformation of POM chain packed in the trigonal unit cell. However, the refinement of the thus-obtained initial structure was difficult since the electron diffraction intensities were modified seriously from the original values by the multiple reflection effect in the single crystal. Rather, the refinement was made successfully by combining the initial structure obtained by the electron diffraction data with the X-ray diffraction data taken for the gamma-ray-polymerized POM multiple crystalline sample, suggesting a usefulness of an organized combination of electron and X-ray diffraction techniques in the structure analysis of polymer crystals.