Thin films of B12+xC3-x have been successfully prepared by the pulsed ion-beam evaporation (IBE) method on glass substrates without substrate heating or sample annealing. B12+xC3-x bulk targets with nominal carbon contents Of x = 0-1.0 were synthesized by spark plasma sintering. Thin films prepared with the targets were found to consist of a B12+xC3-x phase and lattice parameters of the phase were comparable to that of the target. From these results, it has been found that B12+xC3-x thin films with various carbon contents were successfully prepared at room temperature, The thermoelectric properties of the thin films were measured and the B13.0C2.0 thin film exhibited the highest power factor at room temperature among the B12+xC3-x samples reported.