Monolayer Ag-In-Te-Sb-O thin films were deposited by reactive RF-sputtering using Ag8In14Te55Sb23 alloy target in a mixed argon-oxygen plasma at different partial pressure ratio of oxygen to argon (P02/PAr). The optical properties of these films were studied. It was found that films deposited at P02/PAr of 2 to approximately 4% had comparatively large absorption in the wavelength range of 400 - 650 nm. After annealing at 300 degrees Celsius for 30 minutes under protection of argon, the reflectivity in the wavelength range of 500 - 700 nm could rise by about 18 - 25%. The optical constants (n,k) also changed much after heat treatment. XRD analyses indicated that the changes were attributed to the crystallization of Sb. The reflectivity contrast can be as high as 20% after being recorded using short-wavelength laser beam (514.4 nm) with low writing power (10 mW) and short pulse width (100 ns). The film also exhibits certain erasability. This kind of films possess the potentially for use in high density optical storage.