Uniform polyurea (PU) microspheres were prepared through precipitation polymerization of isophorone diisocyanate (IPDI) in water-acetone mixed solvent, under mechanical oscillation and quiescent conditions. Higher yield with better uniformity for the microspheres was achieved under the quiescent process. The preparation was therefore optimized for the quiescent process. The maximal IPDI loading reached 11.0 wt % with the yield, of the microspheres of 88.5%. With acetone replaced by acetonitrile, this yield was increased further to 93.5% combined with also a higher IPDI loading of 15.0 wt % at the same time. The chemical structure of PU was studied using nuclear magnetic resonance. PU microspheres, insoluble in most of organic solvents tested, were dissolved in m-cresol at 30 degrees C and in acetic acid at 66 degrees C. These results showed that the PU microspheres consisted of only linear polymers. This work provides therefore a simple and promising protocol for large-scale production of highly uniform polymer microspheres through precipitation polymerization without any additives.