The production, processing, and transportation of heavy crude oil (HCO) is difficult because of its high viscosity. For practical applications, information on the rheological behavior of HCO plays an important role, especially in flow assurance investigations. In this work, six different imidazolium ionic liquids (ILs) were tested for their effects on the rheological behavior of HCO under high-pressure and high-temperature conditions. The rheological studies were carried out at three different pressures (0.1, 5, and 10 MPa) and four experimental temperatures (298.15, 323.15, 348.15, and 373.15 K). The HCO + IL systems showed favorable viscosity reductions of 26.5% and 31.5% for the systems of HCO + 1-butyl-3-methylimidazolium chloride ([BMIM](+)[Cl](-)) and HCO + 1-octyl-3-methylimidazolium chloride ([OMIM](+)[Cl](-)), respectively, at 298.15 K and 0.1 MPa as compared to the pure HCO system. At 298.15 K and 0.1 MPa, the yield stress of the HCO + IL systems was reduced by about 15-20%, whereas when the temperature was increased to 373.15 K, the yield stress decreased in the range of 25-30% as compared to that of neat HCO. The viscoelastic moduli of the HCO sample at 0.1 MPa, 298.15 K, and about 1.5% strain were found to be G '' (storage modulus) approximate to 11 Pa and G '' (loss modulus) approximate to 7 Pa, indicating that the HCO sample was solidlike, whereas for the HCO + IL systems, the G' and G '' values were reduced to similar to 7 and 3 Pa, respectively. The crossover frequency of the HCO + IL systems was reduced to the range of 25-30% as compared to that of pure HCO. From the various measurements, it was observed that the addition of the ILs to the HCO resulted in improved rheological properties compared to those of the pure HCO system. Further, the results of the microscopic investigation also supported the rheological studies, indicating that the addition of the ILs helped to break the large flocculated structures of HCO into smaller spheres. It was also observed that the IL with the longer alkyl chain length provided greater efficiency in the viscosity reduction with favorable viscoelastic behavior.