Small interfering RNAs (siRNAs) silence the expression of specific target genes by mediating RNA interference (RNAi) in mammalian cells. siRNAs have not only been widely used as a valuable tool for functional genomics research, but they also have demonstrated great potential in biomedical therapeutic applications for diseases caused by abnormal gene overexpression or mutation. One of the most important issues to overcome before full clinical application is the development of effective administration methods for siRNAs to the target tissue or cells in vivo, which is highly dependent on the delivery system. Currently, there are two major kinds of in vivo delivery systems: viral or nonviral. As one of the nonviral carrier systems, nanoparticles, combinations of liposomes and cationic polymer complexes, have exhibited improved in vivo stability, target specificity, and cell/tissue uptake and internalization of the encapsulated RNAi oligos, which result in more effective silencing with less cellular toxicity and immune stimulation. This review will discuss the latest advancements in nanoparticle-mediated RNAi delivery systems, including nano-materials, preparation, and characteristics. In conjunction, the clinical trial cases related to the nanoparticle-siRNA complexes will be highlighted. The safety issues of nanoparticles used in vivo will also be mentioned. Finally, this review will summarize the perspectives for future applications of nanoparticle-mediated RNAi delivery systems.