Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically devastating diseases for the swine industry worldwide. The 372-bp complete nuclear capsid protein (N-protein) encoding gene (ORF7) of 36 field PRRSV isolates from Vietnam collected during 2008-2012 were sequenced and compared with certain vaccines and published strains of PRRSV. The ORF7 nucleotide sequence (nt) similarity and amino acid (aa) identity among 36 strains showed the highest and ranged from 95.1 to 100%. These isolates shared similarities with VR-2332 (nt 91.9-94.3%, aa 92.6-96.7%) and LV (nt 62.7-65.3%, aa 58.5-60.1%). There were higher level of similarity with QN07 (nt 96.2-99.1%, aa 96.7-99.1%) from the 2007 PRRS outbreak in Quang Nam province, CH-1a (nt 93.0-96.2%, aa 92.6-95.9%) isolated in China in 1995 and JXA1 (nt 96.7-99.7%, aa 97.5-100%), the highly pathogenic strain from China isolated in 2006. Six aa mutations located in both variable and conserved regions of N-protein amino acid sequence were detected in most of the 36 isolates and highly pathogenic PRRSV strains in China in comparison to the prototype strain VR-2332. Results of sequence analyses indicated that PRRSVs isolated in Vietnam during 2008-2012 were classified as North American genotype. Phylogenetic tree also clustered those 36 PRRSV isolates, other recently reported Vietnamese strains, highly pathogenic Chinese strains and JXA1-R vaccine strain in the same cluster and separated from the prototype strain (VR-2332) and Ingelvac MLV/Besta vaccine strains. The result on genetic characterization of ORF7 of circulating PRRSV strains may assist the development of effective strategies for monitoring and controlling PRRS in Vietnam.