Abstract Sensitivity and uncertainty analyses of the effective neutron multiplication factor (keff) were conducted for the High Temperature Engineering Test Reactor (HTTR) using the ENDF/B-VII.1 nuclear data and the 44-group covariance library provided in the Whisper-1.1. Amongst the criticality benchmark series, the fully-loaded core was selected and analyzed by using a continuous energy Monte Carlo code, MCNP6.2. The top-five positive sensitivity coefficients (+0.1–+0.03) were found to be U-235 (total nu and fission), C-12 (elastic), graphite S(a,b) (elastic and inelastic), while the top-five negative ones (−0.1–−0.003) were U-238 (n,gamma), U-235 (n,gamma), B-10 (n,alpha), C-12 (n,gamma) and Si-28 (n,gamma). The keff uncertainty originated from the nuclear data was calculated using the previously obtained sensitivity coefficients and the 44-group covariance library. In the present S/U analysis, we obtained the keff uncertainty of about 0.5%. The JENDL-4.0 and ENDF/B-VIII.0 nuclear data were also analyzed for comparison.