In this paper, we report a novel method to synthesize polyethylenimine (PEI) nanogels in the range of 80—200nm with narrow size distribution by photo-chemistry at room temperature in aqueous solution. The nanogels’ size, size distributions and zeta potential were determined by photo correlation spectroscopy (PCS). Spherical morphology of the nanogels was characterized by scanning electron microscopy (SEM) and confirmed by atomic force microscopy (AFM). The nanogels are of high stability, low toxicity and low immunogenicity, as having been confirmed in ,in vivo, tests with mice as animal model, and ,in vitro, tests with human lung and liver cancer cells as well. Efforts are being made for further studies on synthesis and applications of the nanogels.