Lithium ion conducting glasses were prepared in the systems Li6Si2S7-Li(6)B(4)X(9) (X = S, O) by twin-roller rapid quenching. In the system Li6Si2S7-Li6B4O9, composed of both sulfide and oxide, the conductivities at room temperature were kept high and the activation energies for conduction were decreased a little by the doping of 5 mol% Li6B4O9 to Li6Si2S7. The glass stability against crystallization was improved by the doping of 5 mol% Li6B4O9. This improvement in thermal properties was not observed in the sulfide system Li6Si2S7-Li6B4S9. From the comparison of the Si-29 MAS-NMR spectra between Li6Si2S7-Li6B4S9 and Li6Si2S7-Li6B4O9 glasses, it was suggested that the presence of silicon-centered structural units, coordinated with both sulfur and oxygen atoms observed by the doping of a small amount of Li6B4O9, brought about high conductivity and improvement in thermal properties.