Classical swine fever (CSF) is a highly contagious and often fatal disease of swine and wild boars. CSF is characterized by durative high fever and hemorrhage on most organs and lymphatic systems. Infection by CSF virus may run either an acute or a chronic process. In Taiwan, the LPC attenuated vaccine has been used to protect pigs against infection since the 1950s. Although the LPC vaccine is a very efficient vaccine to control the disease, intermittent outbreaks were still observed in field. We consider that the outbreaks of endemic may cause by the virus strains with various levels of virulence, which have ability to present in animals with low titers of antibodies. Basing on these observations, we propose that the studies on viral pathogenesis and virulence are important to understand the control measures. The virus strains obtained from field outbreaks in Taiwan have been classified into distinct genotypes including the Subgroup 3.4 and Subgroup 2.1 based on the E2 and NS5B nucleotide sequences. Different genotypes of viruses will be studied on the characteristics of viral replication in vivo and in vitro respectively. The pathogenesis and antigenesis of various virus strains will explore the characteristics of viral biology for understanding the epidemiology. Furthermore, the specific primers using in the multiplex RT-PCR to differentiate various genotypes of virus strains will be designed. These studies will offer benefit on the eradication of CSF.