Narrowband and high-transmission optical filters are extensively used in color display technology, optical informationprocessing, and high-sensitive sensing. Because of large ohmic losses in metallic nanostructures, metallic filters usually exhibit low transmittances and broad bandwidths. By employing both strong field enhancements in metallic nano-slits and the Wood’s anomaly in a periodic metallic grating, an extra-narrowband and high-transmission metallic filter is numerically predicted in an ultrathin single-layer metallic grating. Simulation results show that the Wood’s anomaly in the ultrathin (thickness H=60 nm) single-layer metallic grating results in large field enhancements in the substrate and low losses in the metallic grating. As a result, the transmission bandwidth (transmittance T>60％) atλ=1200 nm is as small as?λFWHM=1.6 nm, which is smaller than 4％of that in the previous thin dielectric and metallic filters. The corresponding quality factor is as high as Q=λ/?λFWHM=750, which is 40 times greater than that in the previous reports. Moreover, the thickness of our metallic filter (H=60 nm) is smaller than 40％of that in the previous reports, and its maximum transmittance can reach up to 80％. In experiments, a narrowband metallic filter with a bandwidth of about?λFWHM=10 nm, which is smaller than 25％of that in the previous metallic filters, is demonstrated.