The objective of this study is to investigate the effect of Sn content on the microstructure and mechanical properties of theCoCrFeMnNiSnxhigh entropy alloys. The microstructure and mechanical properties of the alloys have been studied by X-raydiffraction, scanning electron microscopy, room-temperature compressive tests and Vickers hardness tests. It has been foundthat Sn has a great effect on the microstructure and mechanical properties of the alloys. The CoCrFeMnNi alloy has a simpleFCC phase structure, while a few white precipitations (MnNi2Sn-like) are observed in the Sn0.03and Sn0.07alloys. With furtheradditions of Sn, the microstructure of Sn0.1,Sn0.3and Sn0.5changes to a typical dendrite structure, the dominated dendriteregion has a lattice of FCC structure which is enriched with Cr, Co and Fe, and the interdendritic region is enriched withMn, Ni and Sn. The volume of the MnNi2Sn-like phase increases with Sn content increasing. The addition of Sn enhancesthe formation of the MnNi2Sn-like phase in the CoCrFeMnNiSnxalloys, and improves the hardness and strength of as-castalloys, but reduces the ductility of the alloys. The strengthening mechanism of alloys was solid solution strengthening andformation of the MnNi2Sn-like phase. In the CoCrFeMnNiSn0.03and CoCrFeMnNiSn0.07alloys, the solid solution is thedominant mechanism. With the Sn content increasing, the second phase strengthening is the dominant mechanism.