Viscosity under high temperatures and pressures is one of the most important thermophysical properties related to the heat-transfer calculation for fuel-cooled thermal management technology. The viscosities of a high-density hydrocarbon fuel JP-10 were measured using a two-capillary method at 242.7-753.3 K under 0.69-6 MPa. After the viscosity data of pure n-octane was measured and compared with the literature data, the measurement system was calibrated, and the average absolute deviation (AAD) and the maximum absolute deviation (MAD) were found to be 0.7196 and 1.35%. Yaws' equation and Bruno's equation are used to correlate the experimental data. The results are that the correlation AAD for Yaws' equation and Bruno's equation are within 4.04% and 4.25% under all the test conditions, respectively.