Single layer Al2O3，Nb2O5 films are prepared by ion beam sputtering deposition (IBSD). Optical constants are measured by infrared variable angle spectroscopic ellipsometer (IR- VASE). Surface topography and roughness of these films are investigated by atomic force microscopy (AFM). 2.7 μm dielectric mirror is prepared using Nb2O5 and Al2O3 as high and low refractive index materials. The environmental stability of single layer films is tested. The results show that asdeposited films have relatively high refractive index, low extinction coefficient, excellent surface morphology and extremely low scattering loss. There is no water absorption in these films which can lead to great increase in extinction coefficient at the 2.7 μm band. The measured reflectance of 2.7 μm Nb2O5/Al2O3 mirror is 99.63% , close to theoretical value. All the films have passed the environmental tests，showing that oxide coatings fabricated by IBSD are stable and durable in hash environment.