Viscosity of rocket propellant (RP-1 fuel) has been measured with a capillary flow technique at high-temperatures and high-pressures. Measurements were made at seven isobars (0.101, 10, 20, 30, 40, 50, and 60) MPa over the temperature range from (301 to 744, up to thermal decomposition temperature) K. The combined expanded uncertainty of the viscosity, pressure, and temperature measurements at 0.95 confidence level with a coverage factor of k = 2 is estimated to be U (eta) = 2%, U (P) = 0.05%, and U (T) = 0.02 K, respectively. The reliability and accuracy of the experimental method and correct operation of the experimental apparatus was confirmed with measurements on pure toluene for two selected isotherms of 583.15 K and 603.15 K at pressures between (10 and 40) MPa. The experimental and calculated values from reference correlation (NIST/REFPROP) for the viscosity of pure toluene show excellent agreement within their experimental uncertainty (AAD is about 0.57% for isotherm of 583.15 K and 1.10% for isotherm of 603.15 K). The measured values of viscosity for RP-1 were compared with the values predicted from a surrogate mixture model (NIST/REFPROP). The significant effect of thermal decomposition (thermal stress) on the measured values of viscosity of RP-1 at high temperatures (above 650 K) was experimentally found.