We report ferromagnetic resonance studies on linewidth of single crystal ultrathin Fe films on GaAs(100). For different orientations of the magnetic field in the film plane, anisotropic linewidth has been observed. An expression for anisotropy of linewidth with isotropic damping was derived and also an expression for anisotropic damping constant was proposed. With these two expressions the data of the angular dependence of linewidth of a 5.2 ML (monolayer) film were fitted well. For the thicker film of 25 ML, the fitting of experimental angular dependence of linewidth could be improved if an inhomogeneous broadening is further considered.