193 nm optical thin films are one of the key components in the 100 nm step and scan microlithography systems.The paper analysized the influence of the optical constants of substrates and thin films, the extinction coefficient of high refractive materials and the surface roughness on the reflectance of HR mirrors. It showed that to obtain a mirror with reflectance of more than 98.5%, the extinction coefficient of the high-refractive material must be below 0.0034 and at the same time, with a surface roughness σ≤1 nm. The influence of the water absorption on the optical performance of the coatings was also analysized and the possible reasons resulted in the difference between the practical and theoretical performance were discussed.