A controlled double-jet reactive crystallization technique using a PEI (polyethylenimine) additive has been conducted to produce agglomeration-free micro-particles of SrSO4 with narrow and mono-dispersed size distributions. PEI successfully inhibits rapid/continuous nucleation due to a complexation with strontium ions before nucleation occurs, and enables realization of perfect separation of nucleation and growth. Our idea to obtain fine particles with desired size distributions will introduce a new potential for contributing to enhance high-valued material properties.