Abstract: To investigate variation of porcine reproductive and respiratory syndrome virus (PRRSV) ORF5 gene, 153 suspected samples of PRRSV were collected from 16 largescale farms of Guizhou province. With RTPCR detecting and ORF5 gene sequencing,results show that among all these samples the homology of ORF5 gene was up to 98.7%—100%, and the homology of amino acids was up to 98.5%—100%;compared with VR2332，CH1a and LV，the homologies of gene sequence were up to 89.2%—90%, 94.8%—95.6%, 59.7%—60.2% respectively, and the deduced amino acid homologies were up to 88.8%—89.3%, 92.7%—94.2%, 54.9%—55.3% respectively. It is indicates that the epidemic strains are from the same origin and belong to Northern American genotype.The ORF5 gene from the variant can be separated into three clusters. The relationship is closed to the highly pathogenic PRRSV that separated at other parts of China,but alienate from PRRSV strain separated early.Our results indicated that the ORF5 gene of PRRSV existed variation.