The kinetics of membrane formation of polyurea microcapsule by interfacial polycondensation was studied both experimentally and theoretically. Diethylenetriamine (DETA) and hexamethylene-1,6-diisocyanate (HMDI) were used as a trifunctional hydrophilic monomer and a bifunctional hydrophobic monomer, respectively, so that a polymer with cross-linked structure was formed. The oil in water (O/W) emulsion was first prepared using water, cyclohexane containing HMDI, and surfactant Tween 85. The emulsion solution was then mixed with DETA aqueous solution to start the interfacial polycondensation. A mathematical model in which DETA transferred to the polymer surface in the water phase, diffused through the polymer membrane, and reacted with HMDI at an interface of the polymer and the oil phase was proposed. The size distribution of microcapsule and diffusion of DETA of both unprotonated and protonated forms were incorporated, and the relation between pH and the concentration of DETA was derived from ionic equilibrium. The calculated results explained well the time variation of the conversion of DETA at a different initial number of moles of DETA and different initial mole ratio of HMDI to DETA. The time variation of the thickness of the polymer membrane was also simulated.