Microalloyed CoCrFeNi high-entropy alloys were produced by magnetron sputtering. In this work, Pd, Ag and Ce were selected as the microalloyed element for CoCrFeNi alloys, respectively. The content of the microalloying element in the high-entropy alloys is less than 1.0 at.%. Via nano-indentation technology, it shows that compared with the high hardness of the unalloyed samples (i.e., 8.5 GPa), the hardness of the microalloyed samples was further greatly improved (e.g., as high as 10~11 GPa). By XRD, face-centered cubic structure was identified in microalloyed CoCrFeNi high-entropy alloys. Nanotwins and nanocrystalline are further characterized by TEM. In addition, the increase in resistivity also confirms the strengthening effects of microalloying in high entropy alloys. The work shows that each high-entropy alloy could be as an alloy base, and may be modified to achieve superior properties by microalloying.