Some K2O-Nb2O5-GeO2 glasses are prepared, and their crystallization behaviors are examined. 25K(2)O.25Nb(2)O(5).50GeO(2) glass with the glass transition temperature T-g = 622degreesC and crystallization onset temperature T-x = 668degreesC shows a prominent nanocrystallization. The crystalline phase is K3.8Nb5Ge3O20.4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630degrees and 720degreesC for 1 h are similar to10 and 20-30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620degrees-850degreesC for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630degreesC for 1 h is estimated to be similar to35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K(2)O.25Nb(2)O(5).50GeO(2) glass change steeply by heat treatment at T-g, i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses.