In recent years, mass outbreaks of highly pathogenic (HP) porcine reproductive and respiratory syndrome virus (PRRSV) have spread all over the Chinese swine industry. Based on the first infectious cDNA clone of HP PRRSV strain pJX143 and that of an attenuated PRRSV, pAPRRS, constructed in our group, we constructed several chimeric clones with various substitutions of structural protein genes (ORF4-7) and 3￠ UTR between attenuated pAPRRS and virulent pJX143.Upon transfection of MA-104 cultured cells, all chimeric constructs pSX12, p5NX12, and p56N12 were rescued. The rescued viruses maintained the similar virological properties, based on the results of the growth curve of the rescued viruses. To test if the chimeric viruses can be used as a vaccine candidate, vSX12 and v56N12 vaccinated pigs were challenged with the HP PRRSV JX143 strain. As a result, the vSX12 vaccinated pigs were all seroconverted by 14-day-post vaccination, while v56N12 vaccinated pigs showed poor antibody response. Upon challenge, the vSX12-vaccinated group showed no signs of clinical PRRS syndrome, and virema period was shorten to 6 days post-challenge. Our results demonstrated that 1) vSX12 chimeric virus is a good vaccine candidate; 2) the virulence determinants of HP PRRSV probably located in coding regions other than ORF3-7 and 3￠ UTR, as our chimeric viruses were proved to be attenuated.