Abstract The objective of this work is to clarify the mechanisms of additional viscosity reduction of heavy oil/bitumen using nano-size metal particles during steam injection techniques. For this purpose, three sets of experiments are designed. The objective of the first series of experiments was to study the effect of metal particles on the viscosity of the produced oil at low temperature. Viscometry experiments at temperatures below 100°C were conducted for this purpose. Then, their effect was studied in the presence of aqueous phase at high temperature of 300°C to simulate the steam stimulation processes. The third set of experiments was designed to study the effect of micro- and nano-sized metal particles on the enhancement of heat transfer within the oil phase. The experiments showed that at low temperatures, the particles reduce the heavy oil viscosity after being mixed with the oil phase. The amount of the viscosity reduction is a function of the concentration of the particles and there exists an optimum concentration of particles yielding maximum amount of viscosity reduction. Also, the trend of viscosity versus concentration of the particles is a function of the type and size of the metal, and the temperature. The second series of experiments revealed that the same trend of viscosity versus concentration of particles is observed at steam injection conditions. However, much higher degree of reduction in viscosity was observed in this case compared to the low temperature experiments. The third series of experiments showed that metal particles used at their optimum concentration do not provide significant improvement of heat transfer. The experiments provided a good understanding of the ongoing mechanisms that would lead to a viscosity reduction by the addition of metal particles. The concentration, type, and size of the particles were found to be highly critical on viscosity reduction. The optimal values of these parameters were identified. The results and observations are expected to be useful in further studies and applications as to the efficiency improvement of the thermal applications for heavy-oil/bitumen recovery.