The equiatomic CoCrFeMnNi Cantor alloy, a face-centered-cubic (FCC) single-phase high-entropy alloy (HEA), has attracted considerable attention owing to its high strength and good ductility over a wide temperature range. The mechanical performance of this alloy was improved by reducing the stacking fault energy (SFE) through composition modification, and thus, a series of near-or non-equiatomic HEAs that are stronger and more ductile than their predecessor have been developed. However, the plastic-deformation behavior and strengthening mechanisms have not yet been fully discovered. In this study, we investigated the yielding and hardening behaviors of the Cantor alloy and FCC-phase Co-rich HEAs with different SFEs by in situ neutron diffraction combined with the first-principles method and electron-microscopy characterizations. The Co -rich HEAs exhibited a higher intrinsic yield strength than the Cantor alloy, mainly because of the larger shear modulus or modulus misfit, and grain refinement being more effective in improving the yield strength of low-SFE HEAs. Furthermore, higher flow stresses and better ductility of the Co-rich HEAs are attributed to the greater dislocation density and a larger number of stacking faults, which enhanced the strain-hardening rate during tensile deformation. The low SFE promoted mechanical twinning, and martensitic transformation contributed to higher strain -hardening rates. The present study provides deep insight into the yielding and hardening of FCC -phase HEAs, the understanding of which is a prerequisite for developing high-performance materials.