To meet the requirement of hydrocarbons online viscosity measurement at high temperatures and high pressures, two different methods are proposed in a two-capillary viscometer based on the Hagen-Poiseuille theory. Referenced flow method (RFM) measures the pressure drop, mass flux, and density ratios of the test and referenced fluid in a two-capillary system, and then calculates the test viscosity from the ratio relations regardless of the tube parameters. In another thermal expansion method (TEM), the fluid viscosity is obtained in a two-capillary process by measuring the pressure drop ratio, density ratio, and thermal expansion of a capillary tube. Note that mass flux and referenced fluid are no longer required in TEM. Pure n-dodecane and a binary mixture of n-heptane and n-octane were adopted respectively to validate the reliability and accuracy of the two methods. Results showed that the average absolute deviation (AAD) was lower than 0.75% and the maximum absolute deviation (MAD) was within 2.2%. Finally, viscosities of two endothermic fuels were obtained using the two-capillary viscometer at temperatures from (303.2 to 673.2) K and pressures up to 5.00 MPa. Accordingly, a viscosity relation formula as a function of temperature for two fuels was given within 4.2% deviation.