The mean diameter of the 100-nm polystyrene sphere was measured with the transmission electron microscope (TEM) with a view to the development of the accurate and visual sizing technique for nm scale particles. To minimize the sizing error due to the inaccurate instrument-stated magnification and the edge location uncertainty, the 300-nm spheres were mixed with 100-nm spheres to provide an internal calibrator, whose diameter is accurately known, The diameters of 100-nm spheres were determined by comparing with that of 300-nm spheres in the same negatives (TEM pictures). For the correction of the electron beam-induced shrinkage effect, the dependence of the shrink-age on the accelerating voltage, beam intensity, and the exposure time was examined and analyzed. Based on such an investigation, and with the several data manipulation routines, the mean diameter was determined with the expanded uncertainty of approximately 2% at the 95% confidence level. The measured value is consistent with those obtained by other laboratories using different techniques for the same spheres.