Glasses with the compositions of xLi(2)O-(70 - x)Nb2O5-30P(2)O(5), x = 30-60, and their glass-ceramics are synthesized using a conventional melt-quenching method and heat treatments in an electric furnace, and Li+ ion conductivities of glasses and glass-ceramics are examined to clarify whether the glasses and glass-ceramics prepared have a potential as Li+ conductive electrolytes or not. The electrical conductivity (sigma) of the glasses increases monotonously with increasing Li2O content, and the glass of 60Li(2)O-10Nb(2)O(5)-30P(2)O(5) shows the value of sigma = 2.35 x 10(-6) S/cm at room temperature and the activation energy (E-a) of 0.48 eV for Li+ ion mobility in the temperature range of 25-200 degrees C. It is found that two kinds of the crystalline phases of Li3PO4 and NbPO5 are formed in the crystallization of the glasses and the crystallization results in the decrease in Li+ ion conductivity in all samples, indicating that any high Li+ ion conducting crystalline phases have not been formed in the present glasses. 60Li(2)O-10Nb(2)O(5)-30P(2)O(5) glass shows a bulk nanocrystallization (Li3PO4 nanocrystals with a diameter of similar to 70 nm) and the glass-ceramic obtained by a heat treatment at 544 degrees C for 3 h in air exhibits the values of sigma = 1.23 x 10(-7) S/cm at room temperature and E-a = 0.49 eV. (C) 2010 Elsevier Ltd. All rights reserved.