Laser additive manufacturing is a direct energy deposition process which manufactures components from 3D model data in progressive layers until a whole part is built as opposed subtractive manufacturing. However, during the procedure, the deposits are subjected to rapid thermal stresses which adversely impact the integrity of the built component. High entropy alloys are materials with complex compositions of multiple elements. Traditionally, these alloys are fabricated using casting and other machining processes, with a recent interest in the use of laser deposition as a possible manufacturing process. To optimize process parameters of high entropy alloys melted on a steel plate, the influence of preheating temperature on the overall quality, microstructure and hardness behaviour of the alloys for aerospace applications were investigated. In this research, 9 samples of AlCoCrFeNiCu and AlTiCrFeCoNi high entropy alloys were fabricated using different laser parameters. The phases, chemical composition, micro-hardness and structural morphologies were characterized with XRD, EDS, Vickers Microhardness tester and SEM respectively before and after preheating the base plates at 400°C. Experimental results show extensive cracking on all the samples without preheating while after preheating all samples were observed to be crack-free. Although, there were no variations on the dendritic structures in the optical micrographs with and without preheating temperature, there were notable changes in the phases and hardness behaviour of the alloys showing that preheating the base plate from 400°C significantly influences the mechanical properties of additive manufactured high entropy alloys and contributes to the elimination of cracks induced by thermal stresses.