The mixed alkali effect on elastic properties of lithium caesium borate glasses 0.25[xLi 2 O-(1-x) Cs 2 O]-0.75B 2 O 3 was studied by Brillouin scattering. The observed mixed alkali effect on the negative deviation from linearity in the composition dependence of longitudinal modulus was discussed on the basis of the decrease of four-fold coordinated boron atoms and the increase of nonbridging oxygens. The temperature dependence of elastic properties was studied in the large temperature range from room temperature to 1500 K to clarify the differences between glasses, crystals and melts. The influence of annealing was clearly observed in LA sound velocity. Fragility was discussed using the temperature dependence of velocity.