Reduction of Birkhoffian systems is studied. First, the differential equations of motion for the Birkhoffian systems are established and their cyclic integrals are given. Second, the Routh functions of Birkhoffian systems are constructed, and the order of the systems is reduced by using the cyclic integrals and the Birkhoffian form made to hold. Finally, an example is given to illustrate the application of the results.