Two-dimensional (2D) materials with fully spin-polarized nodal-loop band crossing are a class of topological magnetic materials, holding promise for high-speed low-dissipation spintronic devices. Recently, several 2D nodal-loop materials have been reported in theory and experiment, such as Cu2Si, Be2C, CuSe, and Cr2S3 monolayers, adopting triangular, tetragonal, hexagonal, or complex lattices. However, a 2D nodal-loop half metal with room-temperature magnetism is still less reported. Here, we report that the 2D Fe4N2 pentagon crystal is a nodal-loop half metal with room-temperature magnetism over 428 K and a global minimum structure via first-principles calculations and global structure search. The Dirac nodal lines in Fe4N2 form a flat nodal loop at the Fermi level and a spin-polarized type-II nodal-loop above the Fermi level, which are protected by mirror symmetry. Our results establish Fe4N2 as a platform to obtain nodal-loop half metallicity in the 2D pentagon lattice and provide opportunities to build high-speed low-dissipation spintronics in the nanoscale.