Dwarf mistletoes are parasitic flowering plants that infect conifers, resulting in substantial loss of growth and mortality. Recently, forest managers in Iran are contemplating whether infection of Juniperus polycarpos C. Koch forests by dwarf mistletoe, Arceuthobium oxycedri(DC.) M. Bieb, influences tree vigor and contributes to insufficient natural regeneration. The present study aimed at assessing the severity of infection and its impact on growth and reproductive output of J. polycarpos. Infected and uninfected trees(n =20 each) were selected for assessment of diameter, height, crown area, and crown volume as well as quantity and quality of cones and seeds. The severity of infection of trees was determined by Hawksworth’s 6-class dwarf mistletoe rating(DMR) system. The DMR system revealed that 40% of the infected sample trees were lightly infected(DMR =1-2) and 60% were moderately infected(DMR =3-4). Growth characteristics did not differ significantly(p > 0.05) between infected and uninfected trees. However, moderate infection affected the reproductive output of J. polycarpos by significantly(p <0.05) reducing the mean number of cones per unit area of the crown, increasing the number of damaged seeds, and reducing seed size and seedgermination capacity. We conclude that reproductive output of J. polycarpos is more sensitive than growth characters to moderate infection by juniper dwarf mistletoe, and this might partly account for poor natural regeneration. Dwarf mistletoes are parasitic flowering plants that infect conifers, resulting in substantial loss of growth and mortality. Recently, forest managers in Iran are contemplating whether infection of Juniperus polycarpos C. Koch forests by dwarf mistletoe, Arceuthobium oxycedri (DC.) M. Bieb, The present study vigor at contribute to to natural regeneration. The present study aimed at assessing the severity of infection and its impact on growth and reproductive output of J. polycarpos. Infected and uninfected trees (n = 20 each) were selected for assessment of diameter, height, crown area, and crown volume as well as quantity and quality of cones and seeds. The severity of infection of trees was determined by Hawksworth’s 6-class dwarf mistletoe rating (DMR) system. The DMR system revealed that 40% of the infected Growth characteristics did not differ significantly (p> 0.05) between infected and infected individuals (DMR = 1-2) and 60% were moderately infected However, moderate infection affected the reproductive output of J. polycarpos by significantly (p <0.05) reducing the mean number of cones per unit area of ​​the crown, increasing the number of damaged seeds, and reducing seed size and seed germination capacity. We conclude that reproductive output of J. polycarpos is more sensitive than growth characters to moderate infection by juniper dwarf mistletoe, and this might partly account for poor natural regeneration.