GJ 367b is a hot, dense, airless sub-Earth orbiting a M dwarf star on a 0.32-day orbit. The James Webb Space Telescope (JWST) observations of the mid-infrared phase curve reveal a zero-albedo planet with no heat recirculation, consistent with a blackbody emission spectrum. The emission spectrum, combined with the white light phase curve, constraints the planet's surface composition and atmospheric pressure. The lack of detectable atmosphere and heat redistribution rules out atmospheres of 1 bar or higher for most compositions, while the emission spectrum excludes thinner atmospheres for some compositions. The planet's high temperature suggests it has lost most of its initial volatiles, possibly due to intense stellar irradiation. GJ 367b is the first sub-Earth with thermal emission observations, providing valuable insights into the atmospheric and surface properties of rocky planets orbiting M dwarfs.GJ 367b is a hot, dense, airless sub-Earth orbiting a M dwarf star on a 0.32-day orbit. The James Webb Space Telescope (JWST) observations of the mid-infrared phase curve reveal a zero-albedo planet with no heat recirculation, consistent with a blackbody emission spectrum. The emission spectrum, combined with the white light phase curve, constraints the planet's surface composition and atmospheric pressure. The lack of detectable atmosphere and heat redistribution rules out atmospheres of 1 bar or higher for most compositions, while the emission spectrum excludes thinner atmospheres for some compositions. The planet's high temperature suggests it has lost most of its initial volatiles, possibly due to intense stellar irradiation. GJ 367b is the first sub-Earth with thermal emission observations, providing valuable insights into the atmospheric and surface properties of rocky planets orbiting M dwarfs.