The vacuum pyrolysis of Iran's used tires and the use of heavy fractions of the derived pyrolytic oils to modify the performance and rheological properties of bitumen were investigated. Two pulverized waste tire samples that are readily available in Iran's market, i.e. Ground tire rubber (GTR) and reclaimed rubber (RR) were subject to vacuum pyrolysis in a bench-scale homemade setup, in the temperature range of 400-600 °C, and at a total pressure of 2.2 kPa. The products of the vacuum pyrolysis process, i.e. Gas, carbon black and pyrolytic oil were utilized to evaluate the yields of vacuum pyrolysis via standard mass balances. The pyrolytic oils were further fractionated via regular batch distillation, and the heavy fractions were characterized using FTIR, HNMR, C 13 NMR, SARA, GPC, and EA techniques. The pyrolysis yield averaged 33.11 wt. % solid residues, 49.83 wt. % pyrolytic oils, and 17.03 wt.% pyrolytic gases, and increased temperature showed increased pyrolytic oils and gases and reduced pyrolytic carbon black (CBp). The characterization results of pyrolytic oils showed similar chemical structures regardless of the feed and temperature, and GPC results indicated that the average molecular weight of the pyrolytic oils was in a range of 250-400 g/mol with a PDI of about 1.5, which is similar to the average molecular weight of typical petroleum cuts. Subsequently the heavy fraction of the pyrolytic oils derived from GTR and RR at 600°C was used to modify the standard and rheological properties of bitumen by simple mixing of bitumen and pyrolytic oil at various proportions. The utilization of heavy fractions of the pyrolytic oil as a modifier of bitumen performance properties showed increased penetration and elastic recovery, and resulted in decreased softening point and Fraass breaking point. Further, the heavy fraction of the produced pyrolytic oil enhanced the high service temperature of pure bitumen by reducing its thermal susceptibility. The addition of 10 wt. % of heavy fraction of the pyrolytic oils demonstrated the best results, and remarkably improved the ductility of pure bitumen. The results of dynamic shear rheology showed that increase pyrolytic oil resulted in decreased cross frequency for storage and loss moduli, which means improvement of the low temperature properties of bitumen. Finally, use of pyrolytic oil in bitumen leads to improved high way low temperature properties of bitumen and good storage stability and aging resistance. Keywords: pyrolysis, bitumen, modification, rheology, ground tire rubber, reclaimed rubber.