Abstract Crystalline phases and second-order optical nonlinearities in crystallized glasses of (Na,K)NbGeO5 (i.e., xNa2O · (25−x)K2O · 25Nb2O5 · 50GeO2) are examined. The molar volume of the glasses decreases with increasing Na2O content, indicating that the glass structure of NaNbGeO5 glass is more compact compared with KNbGeO5 glass. The optical basicity of the glasses is 0.94–0.95, implying that chemical bonds among constituents ions are basically very ionic. (Na,K)NbGeO5 glasses exhibit a unique crystallization depending on the Na2O/K2O ratio. The main crystalline phase in crystallized NaNbGeO5 glass is the NaNbGeO5 phase. KNbGeO5 glass shows a prominent bulk nanocrystallization giving nanocrystals of the K3.8Nb5Ge3O20.4 phase. A glass with the composition of K3.8Nb5Ge3O20.4 also shows a bulk nanocrystallization. The crystallized glasses with K3.8Nb5Ge3O20.4 and NaNbGeO5 crystalline phases do not show a second harmonic generation (SHG) at room temperature. The nonlinear optical (Na,K)NbO3 phases showing a SHG are formed at the surface (∼5 μm) of crystallized glasses with x=15 and 20, giving an orientation of the (1 1 0) plane. The present study suggests the presence of mixed-alkali effect in the crystallization behavior of (Na,K)NbGeO5 glasses.