A broad variation of stacking fault energy (SFE) in high entropy alloys (HEAs) gives rise to rich deformation mechanisms and unique mechanical properties of HEAs. In this paper, we aim to reveal the interplay between grain boundaries and stacking faults in governing the strength and flow stress of CoNiCrFeMn HEA. We carried out atomistic simulations for the tension of polycrystalline CoNiCrFeMn HEA of different grain sizes from 3.0 to 48.6 nm at different temperatures. The tensile flow stress of the HEA follows the traditional Hall-Petch (HP) relation till grain size is down to 15.0 nm. Massive stacking faults contribute to the extra hardening and render a rather gradual drop of flow stress with increasing grain size: In the regime governed by HP relation, partial dislocations emitting from grain boundaries are primarily leading partials; and resulted stacking fault (SF) can serve as barriers to dislocation gliding on intersecting planes. This mild hardening mechanism complements strong hardening from grain boundaries. We expect SF induced hardening could be general in polycrystalline HEAs. The findings reported here may provide a basis and engineering guidance for strengthening and toughening the design of a broad range of HEAs characterized by a wide spectrum of SFEs.(c) 2022 The Author(s). Published by Elsevier Ltd.