Curly top is a destructive disease of beet and several other dicotyledonous plants. The causal agents of this disease in Iran included the severe strain of Beet curly top virus (BCTV-svr) and Beet curly top Iran virus (BCTIV). These viruses are seed-borne in some host plants including Petunia (Petunia X hybrida) and Dianthus barbatus and therefore can affect plant regeneration. To investigate the impact of BCTV-svr and BCTIV infection on flower and seed formation in petunia, seeds were collected from petunia plants showing yellowing and leaf deformation symptoms in the street green sides of college of Agriculture in Shiraz University. Seeds of plants that were infected with one or both viruses were planted. The seedlings developed from the seeds of infected plants along with control healthy seedlings were maintained in a greenhouse under insect-free conditions. When the seedlings reached to the flowering and seed production stages, the number of flowers, capsules and seeds was counted and compared with those of the healthy plants. Statistical analysis using SAS and Excel softwares indicated that the average number of flowers and capsules and finally the seeds of infected plants (either virus singly or mixed infected plants) and healthy (control) plants indicated significant difference (P≤0.01) in these data between the infected and healthy plants. However, there was no significant difference in average number of flowers, capsules and seeds between BCTIV- and BCTV-Svr- infected plants. There was also no significant difference in the aforementioned data between plants infected by either virus singly or both viruses. In another experiment, 20 petunia seedlings were inoculated separately with the infectious clones of either virus by agroinoculation. Five seedlings were inoculated with the LB Broth medium and served as control plants. Three weeks after inoculation and after ensuring the infection of the inoculated plants with viruses, the amount of flowers, capsules and seeds produced in each plant was counted. Comparison of the mean of flower numbers indicated a significant difference (P≤0.01) between plants infected with BCTV-Svr and control plants, but there was no significant difference between BCTIV-infected plants and control plants. The comparison of the average number of capsules showed that there was a significant difference (P≤0.01) between the both virus-infected plants and control plants. Comparison of the mean number of seeds showed that there was a significant difference (P≤0.01) between the plants infected with BCTV-Svr and the control plants, but there was no significant difference between plants infected with BCTIV and control plants. Comparison of mean number of seeds per capsule showed that there was a significant difference (P≤0.01) between the plants infected with each virus and control plants. Generally, the effect of BCTV-Svr on the reduction of flower, capsule and seedling production was more severe than BCTIV.