Polyurea microcapsules were formed by carrying out an interfacial polycondensation reaction in emulsion globules between various diisocyanates and ethylenediamine to investigate the effects of the chemical structure on various characteristics. FTIR showed that very reactive aromatic 2,4-toulene diisocyanate produced many more hydrogen-bonded N-H groups than did other aliphatic diisocyanates. All the microcapsules in this study had prominent thermal stabilities from the results of DSC and TG. It was evident that both the molecular weight and the reactivity of diisocyanates determined the mean diameter and particle-size distribution at the same time.