Potato ( Solanum tuberosum L.) as one of the most important crops is infected by different viruses. Viruses are the most important pathogens of potato around the world and cause yield losses 30 to 50% in single or mixed infections. Isfahan and Chaharmahal Bakhtyari provinces are the major potato producing regionsin Iran. Among potato viruses, Potato virus Y (PVY), Potato virus S (PVS) and Potato leafroll virus (PLRV) are more important due to their widespread and transmission via seed tubers. Despite the economic importance of PVY, PVS and PLRV, there is no information about the rate of PVY, PVS and PLRV transmission through seed tubers. Information about virus transmission through potato tubers is necessary for disease management. So, determination of PVY, PVS and PLRV seed tuber transmission is the main goal of this study. Also, due to importance of synergistic effect of mixed infectionson viral diseases, effects of mixed infection of PVY, PVS and PLRV on symptoms severity and transmission rate through seed tubers were determined. For this purpose, leaves and tubers of potato plants showing different symptoms such as yellowing, mosaic, leaf malformation, leaf rolling and purple coloration of leaf margin were collected from potato fields in Isfahan (Fereydounshahr, Damaneh and Chadegan) and Chaharmahal Bakhtyari provinces (Shahrekord, sefiddasht, Verdijan and Ben). For detection of PVY, PVS and PLRV, total RNA was extracted and RT-PCR was carried out by specific primer pairs of PVY, PVS and PLRV. In RT-PCR, infected potato plants with PVY, PVS, PLRV, PVY/PVS and PVY/PLRV were determined. The seed tubers of infected samples were planted in greenhouse. Symptoms of potato plants were recorded and RT-PCR was carried out for detection of PVY, PVS and PLRV in potato plants. Produced seed tubers from infected potato plants were planted again in greenhouse. Based on RT-PCR results, transmission rate of PVY through seed tubers was estimated 94.4% in second generation and 94.5% in third generation. Transmission rate of PVS was 90.4% and 96.6% in second and third generations, respectively. For PLRV, transmission rate of virus through seed tubers was calculated 100% in second generation and 97% in third generation. Lack of 100% transmission of PVY, PVS and PLRV via seed tubers could be due to unbalanced virus concentration in plants. The potato plants grown from PVY infected seed tubers showed mosaic symptoms in greenhouse. The potato plants grown from PLRV infected seed tubers showed mosaic, yellowing and leaf rolling symptoms. However, PVS infected potato plants were symptomless. Potato plants with mixed infection of PVY/PVS and PVY/PLRV showed more severe symptoms than potato plants with single infection of PVY, PVS and PLRV. To determine synergistic effects of mixed infections, potato variety Santewas mechanically inoculated with single and mixed inocula of PVY and PVS. In addition, potato plants grown from PLRV infected tubers were mechanically inoculated with PVY and PVS to create PLRV/PVY and PLRV/PVS mixed infections. RT-PCR results showed infection of inoculated potato plants a week after inoculation. The first yellowing and mosaic symptoms of inoculated potato plants by PVY/PVS were observed 12 days after inoculation, while inoculated plants with PVY showed mild mosaic symptoms 15 days after inoculation. Inoculated potato plants with PVS were symptomless or had mild symptoms. Symptomsseverity in potato plants inoculated with PLRV/PVY and PLRV/PVS was more than potato plants inoculated with PLRV. Potato plants with mixed infection showed yellowing, mosaic, leaf rolling and purple coloration of leaf margin. Severe symptoms in mixed infections revealed synergistic effect of PLRV/PVY and PLRV/PVS. To confirm synergistic effect in mixed infections, DAS-ELISA was used. The ELISA results showed minor increasing the concentration of PVS and PLRV in mixed infections with PVY. However, PVY concentration was equal in single and mixed infections. Increasing the concentration of VS and PVY was no significant, statistically. High rate of PVY, PVS and PLRV transmission through potato seed tubers shows importance of virus free seed tubers in management of potato viruses. Key words : Potato, Seed tuber, Mixed infection, PVY, PVS, PLRV