Ultradispersed particles of size less than 100 nm for in situ catalytic upgrading have been reported to outperform the augmented catalytic upgrading achieved by incorporating pelleted refinery catalyst to the horizontal production well of the THAI process. Hydroconversion of heavy oil was carried out in a stirred batch reactor at 425˚C, 50 bar (initial H2 pressure), 900 rpm and 60 min reaction time using a range of unsupported transition metal (Mo, Ni and Fe) catalysts. The effect of metal nanoparticles (NPs) was evaluated in terms of product distribution, physical properties and product quality. The produced coke and recovered catalysts were also studied. The levels of API gravity and viscosity of the upgraded oils observed with the NPs was approximately 21˚API and 108 cP compared with thermal cracking alone (24˚API and 53.5 cP), this moderate upgrade with NPs is due to the lack of cracking functionality offered by supports such as zeolite, alumina or silica. However, it was found that the presence of dispersed NPs significantly suppressed coke formation 4.4 wt% (MoS2), 5.7 wt% (NiO) and 6.8 wt% (Fe2O3) compared to 12 wt% obtained with thermal cracking alone. The results also showed that with dispersed unsupported metal NPs in sulfide form the middle distillate (177-343 ˚C) of the upgraded oil was improved particularly MoS2 which gave 50 wt% relative to 43 wt% (thermal cracking) and 28 wt% (feed oil). The middle distillate yields for Fe2O3 and NiO are 47 wt% and 49 wt%, respectively. Hence, iron and nickel-based unsupported NPs showed similar activity when compared to MoS2. The cost and availability of iron-based catalysts compared to Ni and Mo counterpart for heavy oil upgrading are advantages that may justify its preference. Furthermore, the X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis showed that introducing dispersed catalysts to the upgrading helped to produce sponge-type coke which could be used as industrial fuel compared to shot-type obtained upon thermal cracking.