In order to improve mechanical properties of refractory high entropy alloys, silicide was introduced and NbMoTiVSix (x = 0, 0.1, 0.2, 0.3, and 0.4, molar ratio) refractory high entropy alloys are prepared by vacuum arc melting. Phase composition, microstructure evolution and mechanical properties were systematically studied. Results show that the silicide phase is formed in the alloys with addition of silicon, and the volume fraction of silicide increases from 0 to 8.3％with increasing of silicon. Microstructure observation shows that the morphology of dendrite changes from columnar to near equiaxed, eutectic structure is formed at grain boundaries and composed of secondary BCC phase and silicide phase. The average length of the primary and second dendrites decreases with the increasing of silicon. Whereas, the ratio of eutectic structure increases from 0 to 19.8％with the increment of silicon. The refinement of microstructure is caused by heterogeneous nucleation from the silicide. Compressive tests show that the yield and ultimate strength of the alloys increases from 1141.5 MPa to 2093.1 MPa and from 1700.1 MPa to 2374.7 MPa with increasing silicon content. The fracture strain decreases from 24.7％-11.0％. Fracture mechanism is changed from ductile fracture to ductile and brittle mixed fracture. The improvement of the strength is caused by grain boundary strengthening, which includes more boundaries around primary BCC phase and eutectic structure in grain boundary, both of them is resulted from the formation of silicide.