Very high temperature reactors (VHTRs) are planned to be operated between 550 and and demand a thermally efficient intermediate heat exchanger (IHX) in the heat transport system (HTS). The current technological development of compact heat exchangers (CHXs) for VHTRs is at the “proof of concept” level. A significant development in the CHX technologies is essential for the VHTRs to be efficient, cost-effective, and safe. CHXs have very high thermal efficiency and compactness, making them a prime candidate for IHXs in VHTRs. Photochemically etched plates with the desired channel pattern are stacked and diffusion bonded to fabricate CHXs. All plates are compressed at an elevated temperature over a specified period in the diffusion bonding process, promoting atomic diffusion and grain growth across bond surfaces resulting in a monolithic block. The diffusion bonding process changes the base metal properties, which are unknown for Alloy 800H, a candidate alloy for CHX construction. Hence, developing mechanical response data and understanding failure mechanisms of diffusion bonded Alloy 800H at elevated temperatures is a key step for advancing the technology of IHXs in VHTRs. The ultimate goal of this study is to develop ASME BPVC Section III, Division 5 design rules for CHXs in nuclear service. Toward this goal, mechanical performance and microstructures of diffusion bonded Alloy 800H are investigated through a series of tensile, fatigue, creep, and creep-fatigue tests at temperatures 550 to . The test results, failure mechanisms, and microstructures of diffusion bonded Alloy 800H are scrutinized and presented.