Precipitation of monodispersed SrSO4 particles was carried out in the presence of polyethylenimine (PEI). PEI additions varied some particulate properties, e.g., sizes, shapes, size distribution widths, and agglomeration ratios. The present paper mainly focuses on the influence of PEI dosage on crystal size distributions. PEI-assisted double jet techniques require no difficult controls of feeding rates, therefore, this idea will be applicable to precipitation of other inorganic compounds for the purpose of producing monodispersed nano-/microparticles.