Herein, we report the synthesis of (S)-5'-C-aminopropyl-2'-O-methyladenosine and (S)-5'-C-aminopropyl-2'-O-methylguanosine phosphoramidites and the properties of small interfering RNAs (siRNAs) containing four (S)-5'-C-aminopropyl-2'-O-methylnucleosides (A, adenosine; U, uridine; G, guanosine; and C, cytidine). The siRNAs containing (S)-5'-C-aminopropyl-nucleosides at the 3'- and 5'-regions of the passenger strand were well tolerated for RNA interference (RNAi) activity. Conversely, the (S)-5'-C-aminopropyl modification in the central region of the passenger strand decreased the RNAi activity. Furthermore, the siRNAs containing three or four consecutive (S)-5'-C-aminopropyl-2'-O-methylnucleosides at the 3'- and 5'-regions of the passenger strand exhibited RNAi activity similar to that of the corresponding 2'-O-methyl-modified siRNAs. Finally, it was observed that (S)-5'-C-aminopropyl modifications effectively improved the serum stability of the siRNAs, compared with 2'-O-methyl modifications. Therefore, (S)-5'-C-aminopropyl-2'-O-methylnucleosides would be useful for improving the serum stability of therapeutic siRNA molecules without affecting their RNAi activities.