To investigate the dependence of the shell thickness on both the microcapsule size and the concentration of hexamethylene diisocyanate (HMDI) and to analyze the chemical structure of the polyurea shell, polyurea microcapsules were prepared by using the reaction between HMDI uretidione/isocyanurate and ethylene diamine (EDA) in an oil-in-water emulsion. In the experiment, the concentrations of hexamethylane diisocyanate and EDA were changed. The shell thickness was correlated well with the mean size and the concentration of HMDI on the surface area per unit volume of microcapsule. From the solid-state C-13 NMR spectroscopy analysis, the uretidione and isocyanurate ring structures were found to be not changed before and after the reaction. From the wide-angle X-ray diffraction analysis, it was suggested that the chemical structure of the polyurea shell was amorphous. From the FTIR spectra analysis, it could be concluded that polyurea microcapsules were prepared by the reaction between HMDI uretidione/isocyanate and EDA.