This　paper　presents　a　new　kind　of　device　for　generating　the　high　power　terahertz　wave　by　using　a　coaxial　overmoded　surface　wave　oscillator　with　metamaterial　slow　wave　structure　(SWS).　A　metallic　metamaterial　SWS　is　used　to　avoid　the　damage　of　the　device　driven　by　a　high-voltage　electron　beam　pulse.　The　overmoded　structure　is　adopted　to　make　it　much　easy　to　fabricate　and　assemble　the　whole　device.　The　coaxial　structure　is　used　to　suppress　the　mode　competition　in　the　overmoded　device.　Parameters　of　an　electron　beam　and　geometric　structure　are　provided.　Particle-in-cell　simulation　results　show　that　the　high　power　terahertz　wave　at　the　frequency　of　0.14　THz　is　generated　with　the　output　power　255　MW　and　conversion　efficiency　about　21.3%.　(C)　2014　AIP　Publishing　LLC.