PRRS causes severe economic loss for the farmers. Post weaning piglets are highly infected with PRRS because of high sensitivity and low immunity with PRRS. The present research was carried out in post weaning piglets at age from 3 weeks to 6 weeks. The piglets showed deprssion, anorexia, fever, red skin, dyspnea. The piglets also showed congested and hemorrhage lung and pneumonia, and enlarged lymph nodes, hemorrhage kidney. The microscopic less ions consisted of inflammatory infiltration in lung, pneumonia or bronchopneumonia, hemorrhage in pelvis of kidney. The piglets with the above clinical signs and pathological findings were diagnosed to be infected with PRRS by RT-PCR.