When two dissimilar kinds of alkaline ions are used to co-doping oxide glass, a maximum in the electric resistivity or the expansion coefficient appears, that is called the mixed alkali effect. The heat capacity C-p of several rapidly-quenched mixed alkali borate glass was measured using a relaxation calorimeter to investigate the mixed alkali effect. A broad peak of C-p/T-3 was observed with the maximum T-max in the range 5-15 K, which is related to the excess vibrational density of states (VDoS) of a non-Debye nature. Such a peak is called the boson peak and it is also observed as the low-energy peak of inelastic neutron and light scattering experiments. The negative deviation from the linear relation of single alkali borate glasses between the alkali cation size and T-max was clearly observed in a mixed lithium cesium borate glass. The mechanism of this deviation was discussed on the basis of the changes in the BO4 units and the nonbridging oxygens by co-doping. The universal nature of the master plot of C-p/T-3 also shows the deviation from that of pure borate glass by the mixed alkali effect.