Short interfering RNAs (siRNAs) are valuable reagents for sequence-specific inhibition of gene expression via the RNA interference (RNAi) pathway. Although it has been proposed that the relative thermodynamic stability at the 5′-ends of siRNAs plays a crucial role in siRNA strand selection, we demonstrate here that a character of the 2-nt 3′-overhang of siRNAs is the predominant determinant of which strand participates in the RNAi pathway. We show that siRNAs with a unilateral 2-nt 3′-overhang on the antisense strand are more effective than siRNAs with 3′-overhangs at both ends, due to preferential loading of the antisense strand into the RNA-induced silencing complex (RISC). Regardless of the relative thermodynamic stabilities at the ends of siRNAs, overhang-containing strands are predominantly selected as the guide strand; whereas, relative stability markedly influences opposite strand selection. Moreover, we show that sense strand modifications, such as deletions or DNA substitutions, of siRNAs with unilateral overhang on the antisense strand have no negative effect on the antisense strand selection, but may improve RNAi potency. Our findings provide useful guidelines for the design of potent siRNAs and contribute to understanding the crucial factors in determining strand selection in mammalian cells.