Immiscible　bimetallic　composites　are　a　kind　of　transpiration　cooling　material　with　potential　in　high-temperature　service.　Aiming　at　boosting　their　load-bearing　capacity,　the　refractory　phase　was　replaced　with　the　multi-principal　refractory　high-entropy　phase　in　the　present　work.　Bi-phase　metallic　nanocrystalline　NbMoTaW-Cu　composites　were　fabricated　successfully　by　the　powder　metallurgy　method.　The　average　grain　size　of　the　NbMoTaW　phase　in　the　sintered　composite　was　kept　to　be　15　nm.　Two　interfacial　configurations　of　BCC/FCC　and　BCC/amorphous/FCC　were　found　in　the　composite.　Using　atom　probe　tomography,　notable　compositional　inter-diffusion　between　the　immiscible　metals　was　disclosed,　and　the　thickness　of　the　mutual　diffusion　layer　in　the　NbMoTaW-Cu　composite　was　2.2　times　that　in　the　W-Cu　composite.　The　mechanisms　of　entropy　effect　on　the　formation　of　amorphous　configuration　and　interfacial　mutual　diffusion　were　explained　based　on　thermodynamic　calculations.　The　yield　strength　and　Vickers　hardness　of　the　nanocrystalline　NbMoTaW-Cu　composite　are　52%　and　27%　higher　than　those　of　the　W-Cu　counterpart,　respectively.　In　addition,　the　NbMoTaW-Cu　composite　processes　excellent　resistance　to　high-temperature　softening　even　at　900　?.　The　improved　mechanical　properties　were　associated　with　solid　solution　strengthening　of　the　refractory　metal　phase,　as　well　as　the　constraint　effect　and　strengthening　of　interface　between　the　refractory　metal　and　Cu　phases.　This　work　provides　novel　guidance　for　designing　advanced　immiscible　metallic　composites　with　excellent　mechanical　performance.