Co-Mn-Al thin films were prepared using ultrahigh-vacuum magnetron sputtering on thermally oxidized silicon substrates at various substrate temperatures. Composition, crystal structure, magnetic property, and surface roughness of the films were investigated. The film prepared at a substrate temperature (Ts) of around 300 °C had Co2 MnAl B2 structure, revealing partial disorder between Mn and Al sites. Magnetization exhibited a maximum and coercive field exhibited a minimum around Ts =300 °C. Surface roughness increased with the substrate temperature. The film prepared at a substrate temperature of 300 °C was applied to a bottom electrode of a magnetic tunnel junction, thereby creating a large tunnel magnetoresistance.