The Co composition dependences of the structural and magnetic properties of L10-(MnAl)1%xCox alloy films were investigated. The lattice constants of (MnAl)1%xCox films gradually changed with increasing Co content while maintaining the L10-ordered structure below x = 0.08. The saturation magnetization gradually decreased with increasing Co content, and perpendicular magnetic anisotropy was observed below x = 0.08. In addition, Co substitution markedly improved the surface roughness of the films by decreasing the substrate temperature of (MnAl)1%xCox films. We found that both a high magnetic anisotropy and a small surface roughness can be obtained by the substitution of Co atoms into MnAl films.