Refractory high-entropy alloys (RHEAs), particularly NbMoTaW RHEAs, exhibit outstanding softening resistance and thermal stability at ultra-high temperatures, but suffer from room-temperature brittleness, which severely limits their processability and thus practical application. In this study, we successfully achieved large plasticity of >10%, along with high strength of >1750 MPa in the NbMoTaW RHEAs via grain boundary engineering with the addition of either metalloid B or C. It was revealed that the room-temperature brittleness of the as-cast NbMoTaW RHEA originates from the grain-boundary segregation of the oxygen contaminant which weakens grain-boundary cohesion. The doped small-sized metalloids preferentially replace oxygen at grain boundaries and promote stronger electronic interaction with the host metals, which effectively alleviates the grain boundary brittleness and changes the fracture morphology from intergranular fracture to transgranular fracture.