WNbMoTa refractory high entropy alloy (RHEA) with single BCC phase has high strength, high hardness and excellent high-temperature mechanical properties. However, the limited plasticity greatly affects its engineering applications. In this work, WNbMoTaZrx (x = 0.1, 0.3, 0.5, 1.0) RHEAs were prepared by vacuum arc melting. The microstructure, hardness, compressive properties and fracture behavior were characterized. The results have shown that WNbMoTaZrx RHEAs have a single BCC1 solid solution phase at low Zr content, and a second BCC2 phase appeared in the RHEAs with a higher Zr content. With the increase of Zr content, the microstructure of WNbMoTaZrx RHEAs changed to a typical dendrite structure, and simultaneously increase of strength, hardness and compressive plasticity was observed. The improvement of the mechanical properties was mainly attributed to the solid solution strengthening of Zr elements, the formation of more interdendritic regions, and the refinement of dendrite structures. The deformation mechanisms of the RHEAs with increased compressive plasticity were examined using step-by-step observations and discussed related to the evolution of microstructures. The results have shown that under applied loadings, deformation bands were initiated within the dendrite regions, while the propagation of deformation bands was impeded by interdendritic regions, resulting in significantly increased compressive plasticity. The fracture mode also changed from the intergranular fracture in WNbMoTa RHEA to the transgranular fracture in WNbMoTaZr1.0 RHEA. The present work not only gives more insight into the strengthening and deformation mechanisms of the WNbMoTaZrx (x = 0.1, 0.3, 0.5, 1.0) RHEAs, but also explores the application potential of WNbMoTa-based RHEAs.