PM2.5 has been correlated with risk factors for various diseases and infections. It promotes tissue injury by direct effects of particle components. However, effects of PM2.5 on cells have not been fully investigated. Recently, we developed a novel imaging technology, scanning electron-assisted dielectric-impedance microscopy (SE-ADM), which enables observation of various biological specimens in aqueous solution. In this study, we successfully observed PM2.5 incorporated into living mammalian cells in culture media. Our system directly revealed the process of PM2.5 aggregation in the cells at a nanometre resolution. Further, we found that the PM2.5 aggregates in the intact cells were surrounded by intracellular membrane-like structures of low-density in the SE-ADM images. Moreover, the PM2.5 aggregates were shown by confocal Raman microscopy to be located inside the cells rather than on the cell surface. We expect our method to be applicable to the observation of various nanoparticles inside cells in culture media.