A 1,6-α-D-glucan (CPA) with a high molecular weight of 2.0 × 10³ kDa was isolated from the fruits of Castanea mollissima Blume. CPA showed good water solubility, low side effects on normal cells, and moderate cytotoxicity on cancer cells. Selenium nanoparticles (SeNPs) possess significant anticancer effects, but poor water solubility and dispersity. Therefore, an effective nanocomposite was designed in order to enhance the stability and anticancer effects of SeNPs decorated with CPA. Water soluble and dispersed selenium nanoparticles (CPA-SeNPs) were fabricated using CPA and Na₂SeO₃ by redox reaction with ascorbic acid. The average diameter of CPA-SeNPs was 53.7 ± 4.0 with a Se content of approximately 171.4 mg/g. CPA-SeNPs showed a significant anti-proliferative effect on HeLa cells in vitro. Since these NPs could induce apoptosis and S phase arrest. The apoptotic pathway triggered by CPA-SeNPs on HeLa cells was also characterized as induced by reactive oxygen species (ROS) overproduction, mitochondrial dysfunction and activation of caspase-3. This study suggested that CPA-SeNPs could be developed as a potential candidate in cancer treatment, especially against human cervical cancer.