The objective of the present study is to evaluate 2.45GHz and 5.8GHz oven magnetrons, toward a transmitter for space applications. Dc-rf efficiency and frequency spectrum of the oven magnetrons were measured. Significant improvement of efficiency and reduction of back bombardment energy must be necessary for the present 5.8GHz oven magnetron to facilitate thermal management in space and to improve the frequency spectrum, respectively. Thermal vacuum tests of a 2.45GHz oven magnetron were also conducted. From experimental results, the dc-rf efficiency was mainly dependent on magnet temperature. Therefore, thermal control of the magnets and pole pieces is essential for the highly-efficient magnetron operation in space.