How to reduce the bandwidth and insertion loss (IL) of a planar filter at high frequency is challenging work. We investigate the design, simulation, fabrication, and measurement of a terahertz (THz) planar filter with easy integration, low IL, and narrow passband. The direct feeding microstrip filter (DFMF) has a center frequency (f0) of 140 GHz, IL of 2 dB, and 3 dB fractional bandwidth (FBW3dB) of 6%, while traditional coupled feeding microstrip filter with the same configurations has FBW3dB of 20%. The proposed DFMF filter employs direct feeding mode to narrow bandwidth and miniaturized size. The DFMF does not need small coupling gaps in the feeding structures, showing good dimensional insensitivity. Benzocyclobutene is introduced as an interlayer dielectric to reduce IL owing to its controllable ultrathin thickness and low-loss tangent. The introduced planar microelectromechanical system technology is suitable for large-scale fabrication at a low cost. The test results based on through-reflect-line de-embedding technology agree well with the simulation results. The DFMF lays a foundation for realizing the planarization and integration of THz systems.