Diamonds possess significant hardness and durability; however, any attempt to deform them typically leads to brittle fracture. In this study, we comprehensively characterize nanoscale diamond at elevated temperatures, using a combination of experimental and molecular dynamic (MD) simulation approaches. We present the fabrication and compression testing of single crystal diamond < 100 > nanopillars fabricated by electron beam lithography and inductively coupled plasma etching to achieve, for the first time, a highly pristine diamond nanoarchitecture. Remarkably, our findings unveil a distinctive brittle-to-plastic transition in diamond behavior, occurring at approximately 550 °C. The fracture stress exhibited by these pristine nanopillars exceeds the strength reported previously in literature for < 100 > oriented diamond. Complementary MD simulations provide a deeper understanding of the underlying deformation mechanisms involved in brittle-to-plastic transition. The insights from this study offer novel pathways for advancing both the fabrication of pristine diamond nanoarchitectures and their strain engineering, with envisioned extreme high-temperature applications in diamond based micro- and nano- electromechanical systems (MEMS/NEMS) and micro/nanomechanics.