X-ray　luminescence　optical　imaging　has　been　recognized　as　a　powerful　technique　for　medical　diagnosis　due　to　its　deep　penetration　and　low　auto-fluorescence　in　tissues.　However,　the　low　luminescence　efficiency　of　current　X-ray　luminescence　nanoprobes　remains　a　major　hurdle　for　sensitive　bioimaging　in　practical　medical　applications.　Here　we　present　a　new　kind　of　energy　transfer-sensitized　X-ray　luminescence　nanoprobe　(PEG-NaGd(WO4)2:Eu)　for　highly　effective　optical　bioimaging.　Under　X-ray　excitation,　the　tungstate　host　absorbs　the　X-ray　photons　and　then　transfers　the　energy　to　the　Eu3+　luminescence　center,　thus　enhancing　the　luminescence　efficiency　of　the　nanoprobes　for　high　sensitivity　optical　in　vivo　imaging.　Moreover,　the　shortened　T1　relaxation　response　of　Gd3+　ions　and　X-ray　attenuation　capability　of　W　atoms　enable　the　nanoprobes　to　serve　as　efficient　contrast　agents　for　magnetic　resonance　imaging　(MRI)　and　computed　tomography　(CT)　imaging.　Therefore,　combined　with　the　MRI,　CT　and　X-ray　luminescence　imaging　capabilities,　the　present　PEG-NaGd(WO4)2:Eu　nanoprobes　could　be　used　as　promising　multimodal　imaging　contrast　agents　in　biological　systems.　©　The　Royal　Society　of　Chemistry　2018.