The therapeutic potential of quercetin-loaded Pluronic F-127 (PF-127)/Tween 80 mixed nanomicelles as a passive targeted drug delivery system for breast cancer therapy is evaluated. Quercetin-loaded mixed nanomicelles with different mass ratios of drug/PF-127/Tween 80 were prepared by thin-film hydration method. The physical interactions of quercetin with PF-127 and Tween 80 in an optimal ratio were investigated. The results of in vitro drug release experiments showed that the mixed micellar system exhibits a prolonged and sustained release behavior compared to the solution of free quercetin. The in vitro cytotoxicity studies of quercetin-loaded mixed nanomicelles on breast cancer cells revealed that the encapsulated drug has a lower half-maximal inhibitory concentration (IC50) value compared to the free drug.