Classical swine fever (CSF) is a highly contagious disease of pigs, which causes important economic losses worldwide. In the present study, the specific effect of RNA interference on the replication of CSF virus (CSFV) was explored. Three species of small interfering RNA (siRNA), targeting different regions of CSFV Npro and NS5B genes, were prepared by in vitro transcription. After transfection of PK-15 cells with each of the siRNAs followed by infection with CSFV, the viral proliferation within the cells was examined by indirect immunofluorescence microscopy. At 72 h post-infection, only a few siRNA-treated cells were positive for viral antigen staining, while most untreated virus-infected cells were positive. Treatment with the siRNAs caused a 4-12-fold reduction in viral genome copy number as assessed by real time RT-PCR. Transfection with the siRNAs also suppressed the production of infectious virus by up to 467-fold as assessed by TCID50 assay. These results suggested that the three species of siRNAs can efficiently inhibit CSFV genome replication and infectious virus production, with the inhibition persisting for 72-84 h.