Classical swine fever and porcine reproductive and respiratory syndrome are both notifiable diseases of the World Organization for Animal Health (OIE). The two diseases exhibit indistinguishable clinical symptoms and sometimes co-exist in swine herds. In this study, a duplex real-time RT-PCR for simultaneous detection of Classical swine fever virus (CSFV) and North American (NA) genotype Porcine reproductive and respiratory syndrome virus (PRRSV) based on two differently labeled TaqMan probes was developed and evaluated. The detection limit of the assay was 3.2 TCID(50) or 13 RNA copies for CSFV and 1.8 TCID(50) or 10 RNA copies for PRRSV, about 50 times more sensitive than conventional RT-PCRs. The duplex real-time RT-PCR was capable of specifically detecting different subgroups of wild-type CSFV and different strains of NA-genotype PRRSV, whereas a number of non-CSFV/PRRSV porcine viruses and bovine pestivirus were tested negative. Out of 155 field samples, 16 were tested positive for CSFV, 73 were positive for PRRSV, and 13 were co-infected with the two viruses. These results were 99.4% in agreement with those using conventional RT-PCRs. Therefore, the assay provides sensitive and simultaneous detection and differentiation of CSFV and PRRSV.