A formation mechanism of guava-like polymer/SiO2 nanocomposite particles (NCPs) in in situ emulsion (co) polymerization systems based on the enhanced homogenous nucleation was proposed in the present paper. The reactive 3-(trimethoxysilyl) propyl methacrylate modified SiO2 nanoparticles (NPs) were chemically linked to form a large amount of SiO2 clusters at the beginning of polymerization. The formation of the SiO2 clusters driven by highly efficient grafting reaction of monomers to SiO2 NPs could be regarded as an enhanced homogenous nucleation process. The SiO2 clusters functioned as the main polymerization loci to form guava-like polymer/SiO2 NCPs in the following polymerization process. The morphological evolution of the polymer/SiO2 NCPs, and the role of the grafting reaction and nucleation pattern in the formation of the guava-like polymer/SiO2 NCPs were systematically investigated.