A High Temperature Gas-cooled Reactor (HTGR) system was generally designed and planned to be constructed in a medium size reactor (around 600 MWe power output) in the past. In this paper, authors are exploring the potential of very small modular type HTGR with a special attention given to the power conversion system. Since HTGR is relatively less challenging to achieve passive safety while easy to adopt an air-cooled Brayton cycle for a power conversion system, HTGR can be suitable for a small modular reactor application. However, as the size of the power system decreases the consisting component performance usually degrades and the final effect on the total system performance was not seriously studied before for a small size HTGR system. 20 MWth reactor was chosen for this study to investigate how helium Brayton cycle performance can be affected by the component level performance and how the optimal operating condition shifts when the system size reduces. The discussion of component design that can deliver assumed performance for the cycle estimation will be briefly presented as well. Furthermore, a supercritical CO2 cycle option will be compared to the helium Brayton cycle to show that the S-CO2 cycle can be a good alternative for a very small scale system.