Abstract: The changes in viscosity characteristics of fuel oil and coal-water slurry fuel as well as biofuel precursors-polysaccharide aqueous solutions with the addition of nanoparticles-carbon nanotubes have been considered. A theory describing the experimental data and predicting the existence of reduction in the viscosity over a range of nanoparticle concentrations has been presented. It was shown that one of the reserves for hydrocarbon fuel rheological property improvement is the determination of nanoparticle concentration leading to viscosity reduction of a certain type of liquid fuel at their operating temperatures and the use of nano-additives within the required concentration range.