The aim of the project is to establish a RT-PCR method to differentiate field isolates and lapinized vaccine strains of classical swine fever virus from tissue samples. This invention aims at the nucleotide-sequencing difference of Lapinized swine fever vaccine virus and field isolate of CSFV in the characteristic of the 3'-untranslated region on the nucleus of the CSFV lapinized vaccine strain, which comprises a 12-13 bp nucleotide (poly T) insert in comparison with the field isolate of CSFV. By a specially-designed pair of primers at both ends of that insert and by using the CSFV to perform the nucleotide expansion methods, such as RT-PCR or n-PCR, to separate the PCR products by electrophoresis for direct analysis of the result. The commercialization of diagnostic kit for differential diagnosis CSFV will be prepared and 200 field samples will be tested with the diagnostic kit.