Cancer cells are commonly characterized by high cellular oxidative stress and thus have poor tolerance to oxidative insults. In this study, we developed a nano-formulation to elevate the level of reactive oxygen species (ROS) in cancer cells via promoting ROS production as well as weakening cellular anti-oxidizing systems. The nanoagent was fabricated by encapsulating two natural product molecules, cinnamaldehyde (CA) and diallyl trisulfide (DATS), in PLGA–PEG copolymer formulated nanoparticles. CA promotes ROS generation in cancer cells and DATS depletes cellular glutathione. CA and DATS exhibited a synergistic effect in amplifying the ROS levels in cancer cells and further in their combined killing of cancer cells. The in vivo experiments revealed that the CA and DATS-encapsulated nanoagent suppressed tumors more efficiently as compared with the single drug-loaded ones, and the tumor-targeted delivery further enhanced the therapeutic efficacy. This study suggests that the combined enhancement of oxidative stress by CA and DATS could be a promising strategy for cancer therapy.