We demonstrate a newly-developed magneto-fluorescent carbon nanotube (CNT)-mediated siRNA (CNT-siRNA) delivery system, which significantly silences our target of interest, gastrin-releasing peptide receptor (GRP-R), in neuroblastoma. CNT-siGRP-R resulted in a 50% silencing efficiency and a sustained efficacy of 9 days for one-time siRNA treatment in vitro, whereas siRNA delivered by the commercial transfection reagent couldn't knockdown GRP-R expression. We further show that CNT-siRNA efficiently inhibits the growth of subcutaneous xenograft tumors in vivo. This system allows us to track the CNT-siRNA distribution via both near-infrared fluorescence and magnetic resonance imaging. Moreover, our delivery system can be used to knockdown GRP-R expression in other cancer cell types, such as human breast cancer cells. The high efficiency and sustained efficacy may indicate that the natural stacking interactions between CNTs and siRNAs can protect siRNAs from degradation and enhance their stability during the delivery process.