CoNiV is a novel medium-entropy alloy with excellent mechanical properties. Currently, alloying CoNiV with Al has been extensively employed to improve its mechanical strength. Unfortunately, the microstructure of CoNiV changes from a single phase to a dual phase due to the addition of Al, which considerably reduces its corrosion resistance. Therefore, developing new strategies to improve its mechanical properties is imperative. In this study, the microstructure and static tensile mechanical properties of (CoNiV)_{100 - x} B _x alloys (x = 0, 0.1, and 0.2, atomic fraction, %) are systematically investigated. The results revealed that the strength and ductility of CoNiV can be significantly improved by doping a small amount of B. With the introduction of 0.2%B, the yield strength, ultimate tensile strength, and elongation of CoNiV are improved, increasing by 12%, 10%, and 30%, respectively. The crystal structure, grain size, crystallographic orientation, and plastic deformation mechanism of CoNiV are not affected due to microalloying with B. At room temperature, (CoNiV)_99.8B_0.2 exhibits fcc structure. The plastic deformation mechanism during static tensile deformation is manifested as dislocation slip, while martensitic transformation and twin effects induced by stress are not observed. The results of the nanohardness tests indicated that doping with trace amounts of B could remarkably enhance the grain/twin boundary hardness, confirming the grain/twin boundary strengthening effect of B on CoNiV. The strengthening of grain/twin boundaries leads to increased resistance of dislocations and provides the ability to hinder crack expansion, resulting in the simultaneous enhancement of the strength and ductility of (CoNiV)_99.8B_0.2. Moreover, the B element dissolved into the matrix would serve as a pinning site for dislocation, thus contributing to the increased strength of CoNiV.