A conceptual design study of the Korean fusion demonstration reactor (K-DEMO) has been carried out since 2012 as following the Korean fusion development roadmap based on the Fusion Energy Development Promotion Law (FEDPL) legislated in 2007. The present in-vessel components of the K-DEMO have been studied for the 2,200 MW of fusion power. Almost half of the plasma power should be cooled down in the divertor. Especially, 10 MW/m2 of the peak heat flux locally concentrates on the narrow section of the divertor target. To faithfully cool down the heat load, the selection of materials that the divertor is important as well as the decision of design parameters. Especially, the choice and design of the heat sink material in the divertor target are quite significant because the heat sink directly interfaced with the coolant. Reduced activation ferritic martensitic (RAFM) steel and CuCrZr have been considered the most promising candidates as the heat sink material. The preliminary designs of the high heat flux (HHF) units operating within materials’ own allowable temperature were derived by accomplishing thermohydraulic analyses for RAFM and CuCrZr. Based on the designs of HHF units with a support structure.