To investigate the unexplored central region of multicomponent alloy phase space, high entropy alloys, as a new class of materials, have been studied in last 12 years. HEAs are nearly equi-atomic and multi-principal element system, which can crystalize as a single phase. Multi-phases HEAs were also developed by certain methods recently. To improve mechanical properties of HEAs, friction stir processing, as a solid state microstructural modification method, has been used in this study. Cast Al0.1CoCrFeNi HEA which has single FCC phase was processed by FSP. FCC phase grains were refined significantly. In addition, aluminum powders and Y2O3 nano-particles were added during FSP, separately. For FSP with Al powders, in-situ AlNi phase was formed in the FCC matrix. For FSP with Y2O3 nano particles, grains were refined further by in-situ Y2O3 compared with FSP of single phase HEA. Also, Y2O3 nano particles enhance mechanical properties by oxide dispersion strengthening mechanism.