Both Alloy 617 and Alloy 230 have been considered the most promising structural materials for the Very High Temperature Reactor (VHTR). In this study, mechanical properties of both alloys were examined by performing tensile tests at three different strain rates and at temperatures up to 1000°C. This range covers time-dependent (plasticity) to time-independent (creep) deformations. Strain-rate sensitivity analysis for each alloy was conducted in order to approximate the long-term flow stresses. The strain-rate sensitivities for the 0.2% flow stress were found to be temperature independent (m [approximate] 0) at temperatures ranging from room temperature to 700°C due to dynamic strain aging. At elevated temperatures (800°C to 1000°C), the strain-rate sensitivity significantly increased (m > 0.1). Compared to Alloy 617, Alloy 230 displayed higher strain-rate sensitivities at high temperatures. This leads to lower estimated long-term flow stresses. Results of this analysis were used to evaluate the current American Society of Mechanical Engineers (ASME) allowable design limits for each alloy. The study showed that the allowable design stresses in the ASME Boiler and Pressure Vessel Code for either alloy do not provide adequate long-term degradation estimation. Nevertheless, rupture stresses for Alloy 617, developed in the ASME code case N-47-28, can generally satisfy the safety margins at 800°C and 1000°C estimated in the study following the strain-rate sensitivity analysis. Furthermore, additional material development studies might be required, since the design parameters for rupture stresses are constrained such that the current VHTR conceptual designs cannot satisfy the material limits.