A two-capillary method based on the Hagen-Poiseuille theory was proposed for on-line measurements of dynamic viscosity of hydrocarbons at high temperatures and high pressures. A two-capillary viscometer was designed and proved to be applicable to the viscosity measurements in a mini-tube. The standard uncertainty of dynamic viscosity measurements is identified as (2.22-5.28) % (coverage factor k=2). N-heptane, n-octane and their binary mixture were tested respectively in the calibration experiments of the viscometer. Results showed that the measured maximum relative deviations of all substances were all within 2% and the average absolute deviations were within 0.72%. Finally viscosities of a kerosene kind hydrocarbon fuel were measured using the two-capillary viscometer at temperature from (303.2 to 513.2) K and supercritical pressures of (3.00 and 4.00) MPa. (C) 2015 Elsevier B.V. All rights reserved.