SnOP 2 O 5 glasses have high refractive index and low glass transition temperature but have poor water durability. To improve water durability, NbO 2.5 was added to SnOP 2 O 5 glasses, preparing SnONbO 2.5 P 2 O 5 glass. It was found that addition of only 4 mol % NbO 2.5 was enough to achieve significant improvement of water durability, and at the same time, no degradation in thermal and optical properties was observed.