Small-interfering RNAs (siRNAs) execute specific cellular gene silencing by exploiting the endogenous RNA interference (RNAi) pathway. Therefore, excess amounts of siRNAs can saturate cellular RNAi machineries. Indeed, some siRNAs saturate the RNA-induced silencing complex (RISC) and competitively inhibit silencing by other siRNAs. However, the molecular feature of siRNAs that specifies competition potency has been undetermined. While previous reports suggested a correlation between the competition potency and silencing efficiency of siRNAs, we found that the silencing efficiency was insufficient to explain the competition potency. Instead, we show that the nucleotide sequence of the 5'-half of the guide strand determines the competition potency of an siRNA. Our finding provides important information for understanding the mechanistic basis of competition in combinatorial RNAi treatment.