In this work, nanocatalysts as Ni, Mo, Fe nanoparticles (NPs) were synthesized by the modi fied inverse mi- croemulsion method at room temperature conditions in order to apply them to heavy crude oil (HCO) in situ hydroprocessing and to improve their physicochemical properties. All samples of NPs were characterized by X- ray photoelectron emission spectroscopy (XPS), X-ray energy dispersion spectroscopy (EDX), small -angle X-ray scattering (SAXS), and scanning transmission electron microscopy (STEM). Catalytic evaluation of nanocatalysts was performed in a batch reactor at 372 ?C, 9.8 MPa of initial H 2 pressure and 1 h of reaction time. The reaction products were studied by analysis of saturates, aromatics, resins, and asphaltenes (SARA) fractions, simulated distillation (SIMDIS), API gravity, and both nitrogen and sulfur contents to determine their physicochemical properties. Applied techniques revealed that nanoparticles of Ni, Mo, and Fe with spherical morphology with average sizes in the range (2.5 -20) nm and agglomerates smaller than 50 nm were obtained. The nanocatalysts synthesized improved from 13 to 18?API, also of an asphaltenes conversion in the range 20 -43%, and moderate sulfur and nitrogen removal were obtained. SARA analysis indicated that saturated and aromatics fractions increased while resins and asphaltenes were reduced, increasing the possibility of a high mobility of crude oil into the reservoir.