A preliminary conceptual design of a reactor cavity has been carried out for a heavy ion fusion reactor system, “HIBLIC-I” [1]. Design efforts have been mainly concentrated on the feasibility of the physical scenario adopted in the design and also on the system integration of structures and components into a compact reactor cavity. The design features of the reactor are summarized as follows: A compact reactor cavity is surrounded by a sacrificial wall protected against target explosions by a cylindrical lithium fall (Li curtain), which later flows upward between the sacrificial wall and a coaxial cavity wall to form a thick lithium layer which serves as a tritium breeding blanket. This configuration makes it possible to raise the outlet coolant temperature up to 500°C with the saturation temperature of lithium in the cavity being less than that corresponding to the pressure of 10−2 Pa necessary for heavy ion beam transmissions. The sacrificial and cavity wall have different design lives of 2 (scheduled replacement) and 30 years (reactor life), respectively, which are confirmed from neutron calculations. The sacrificial wall is placed sufficiently apart from the lithium curtain to reduce a dynamic loading generated by the curtain disruption. (The pressure generation in the thick lithium layer due to the neutron irradiation is avoided by introducing noncondensable gas bubbles or compressible substance.) From the viewpoint of better compatibility with flowing high temperature lithium, a ferritic steel, HT-9, instead of stainless steel is employed as the structural material, and cold and hot traps are installed to control impurities in the liquid lithium using monitoring and sampling devices. The neutron calculation gives a high breeding gain of tritium in the present blanket. For a recovery of tritium from the liquid lithium, a tritium trapping system with yttrium solvent is proposed and integrated into the system.