Constitutive activation of signal transducer and activator of transcription 3 (Stat3) has been observed in many human malignancies. Using the sequence-specific RNA interference (RNAi) method to switch off Stat3 expression, it may be possible to arrest cancer growth. In this study, we aimed to identify the most effective sequence of a synthetic small interfering RNA (siRNA) specific for Stat3 (Stat3-siRNA) and the effect of Stat3 suppression on the growth of oral squamous cell carcinoma cells. Ten designed siRNAs with known sequences were screened for the best RNAi effect at the working concentrations of 1 and 10 nM. The range of reduction of Stat3 expression varied from 21 to 67% for 10 nM siRNAs, and from 13 to 73% for 1 nM siRNAs. Three out of the 10 screened siRNAs reduced Stat3 expression to lower levels compared with the GFP-siRNA control. The interferon response of some siRNAs was observed at a concentration of 10 nM. However, at 1 nM, the mRNA levels of interferon response genes (OAS1, OAS2, MX1 and ISFG3gamma) remained unchanged. The growth of GFP-SAS, HSC-3, HSC-4 and KB cells was strongly inhibited by the use of three effective Stat3-siRNAs in comparison with other Stat3-siRNAs and GFP-siRNA. Moreover, the mRNA levels of genes for which transcription is activated by Stat3 were markedly suppressed. These results suggest that targeting Stat3 using siRNA may constitute a useful approach for the treatment of oral squamous cell carcinoma.