The error of the two-layer antireflective coating is analysed. It is shown that optical properties of the film composed of non-quarter wavelength stack is stabler than those of the film composed of quarter wavelength stack. The two-layer antirefleetive film has been mads on K9 substrate and Ta2OB5 and SiO2 used as coating material. Its reflectivity at one surface is lower than 0.03% and laser induced damage threshold is over 7 GW/cm2 (at 1.06 μm and 1 ns).