Majority of Thai swine herds are PRRS virus positive resulting and negative replacement gilts are not available commercially resulting in PRRS virus eradication through total depopulation and repopulation with negative replacement, and herd closure not possible. The objective of the study was to produce PRRSV negative replacement stock from PRRSV positive herd. The study was conducted in 3 phases. The phase I was to investigate factors associated with the production of negative replacement stock from PRRSV positive herd. The Phase II was to produce negative replacement stock and the Phase III was to produce the PRRSV negative herd. The study was performed in a Great grand parent (GGP) and Grand parent (GP) herd with an inventory of 1,200 sows operating one-site production system, which sows herd, nursery and finishing pigs are in the same site. The herd previously experienced sever PRRSV outbreak. Blood was collected from 100 multiparous sows at 2 weeks prefarrowing and assayed for antibody by ELISA. Blood was collected from pigs farrowed from the sows at 1 and 3 weeks of age and assayed for antibody by ELISA and virus by PCR. All antibody and PCR results and sow data including parity and maternal line were analyzed using linear regression model. The results demonstrated that the production of negative pigs related to infection status of pigs and maternal line, but not related with antibody level, both in prefarrowing sows and pigs. The method developed from the Phase I was used to produce pigs for the Phase II. Prior to conduct the study nursery was depopulation and herd was closed from gilt introduction for 2 month. The results of the Phase II demonstrated the production of negative pigs. If pigs are PRRSV negative, they will remain serologically negative until becoming gilt replacement. Sentinel was used to detect the shedding status of the sow herd 2 month prior to negative stock introduction. The negative gilt introduction occurred following sentinel remaining serologically negative. The positive sow herd was replaced with negative replacement stock at 60% per year and become serologically PRRSV negative within 18 month.