To reduce nucleator’s size and improve its dispersion, microemulsions of sodium 2,2’-methylene bis-(4,6-di-tert-butylphenyl) phosphate (NA11) and NA11/sodium benzoate (SB) composite nucleator are prepared by choosing ethanol as NA11’s solvent and water as SB’s solvent. In the microemulsions based on ethyl butyrate (oil phase), ethanol (cosurfactant), Tween 80 (surfactant) and water, NA11 nucleator within the ethanol are dispersed on the interface of droplets, whose size is ca. tens of nanometers as characterized by dynamic light scattering and cryo-TEM. Investigations on PP’s crystallization behavior by DSC and POM indicate NA11’s nucleation efficiency can be further improved using microemulsion method. However, the blank microemulsion can adversely affect PP’s crystallization properties due to its residual oil and surfactant components in PP matrix. The NA11/SB composite nucleator contained in microemulsion seems to be better dispersed with the help of microemulsion than the original nucleator powder, as more nuclei can be found in the POM photograghs of sample modified with NA11/SB composite microemulsion. The flexural modulus of virgin PP decreases after introducing the blank microemulsion owing to the negative influence of residual oil and surfactant components within the microemulsion on the stiffness of PP. Compared to the blank microemulsion, the microemulsions loaded with nucleators result in higher stiffness. A synergistic effect is found in NA11/SB composite microemulsion, which shows smaller size of dispersed phase and better improvement to PP’s stiffness, which may be attributed to a better dispersion and smaller size of NA11 within the composite microemulsion. A possible dispersion mechanism of nucleator microemulsions in the final PP matrix is proposed according to the microemulsions’ structure and their nucleation effects.