Microstructure and tensile behavior of CoCrFeNi 2.1 Nb x (x = 0.2, 0.4) high entropy alloys were investigated in the as-cast and aged (800 °C/24 h) conditions. Increasing Nb addition increased strength but reduced ductility due to the increased volume fraction of Laves phase. On aging, fine acicular precipitates formed throughout the matrix in the CoCrFeNi 2.1 Nb 0.4 alloy, but only at the dendrite boundaries in the CoCrFeNi 2.1 Nb 0.2 alloy. While both the aged alloys showed increase in strength due to the presence of the fine precipitates, the favorable microstructure of the CoCrFeNi 2.1 Nb 0.2 ensured a larger mean free path for gliding dislocations, thus rendering superior strength-ductility synergy. © 2019 Elsevier B.V.