Small non-coding RNAs regulate gene expression at posttranscriptional level in eukaryotic cells. Two classes of such small (~21-25 nt) RNAs that have been extensively studied in gene silencing are short interfering RNAs (siRNAs) and microRNAs (miRNAs). RNA interference (RNAi) is process whereby double-stranded RNA induces the sequence-specific degradation of homologous mRNA. The RNAi machinery can also be programmed in human cells by introducing 21-nt siRNA duplexes that are assembled into RNA-induced silencing complexes (RISC). In this dissertation, systematic analysis of siRNAs with deletions at the passenger and/or guide strand reveals that a short RNAi trigger, 16-nt siRNA, induces potent RNAi in human cells. The 16-nt siRNA more effectively knocked down mRNA and protein levels than 19-nt siRNA when targeting the endogenous CDK9 gene. In vitro kinetic analysis of human RISC indicates that 16-nt siRNA has a higher RISC-loading capacity than 19-nt siRNA. These results suggest that 16-nt duplexes can be designed as potent triggers for RNAi. RISC can be programmed by small interfering RNAs (siRISC) to cleave a perfectly complementary target mRNA, or endogenous microRNAs (miRISC) to inhibit translation by binding imperfectly matched sequences in the 3’-untranslated region (3’-UTR) of target mRNA. Both RISCs contain Argonaute2 (Ago2), which localizes to cytoplasmic mRNA processing P-bodies. This dissertation shows that RCK/p54, a DEAD box helicase, interacts with Ago2, in affinity-purified active siRISC or miRISC, facilitates formation of P-bodies. Depletion of RCK/p54 disrupted P-bodies and dispersed Ago2 throughout the cytoplasm, but did not significantly affect siRNA-mediated RNAi. Depleting RCK/p54 releases general and miRNA-induced translational repression. These findings imply that miRISC-mediated translation repression requires RCK/p54, also suggest that location of miRISC to P-bodies is not required for miRNA function, but is the consequence of translation repression. To elucidate the function of RCK/p54 in miRNA-mediated gene silencing, analysis of a series of YFP-tagged RCK/p54 mutants reveals the motif required for P-body localization, interaction with Ago2, and/or facilitating the miRNA-mediated translation repression. Additionally, rabbit reticulocyte lysate system was used to recapitulate the miRISC function in a cell-free system and confirmed the requirement of RCK/p54 for miRNA function in vitro. Analysis of Ago2 distribution in the polysome profiling in RCK/p54-depleted cells, compared to that in normal cells, revealed that RCK/p54 facilitates miRISC by trapping it at translation initiation complex. These data suggest that interaction of RCK/p54 with Ago2 is involved in the repression of translation initiation of miRNA function.