Abstract: A co-aperture dualband waveguide slot array antenna design method is proposed by studying co-aperture dualband array antennas. This method is suitable for designing a high performance antenna composed of two ridge waveguide slot arrays operating in different bands. This antenna is especially suitable for small dual band radars. This paper describes in detail how to reduce the effect of mutual interference between two bands on antenna performance. Finally, some design and test results of Ka/Ku antennas are given. The test results are in good agreement with the simulation results, verifying that the design method is correct.