The technology of transform from microstrip to waveguide at Ka band is researched. The transform is achieved by ridge waveguide and glass seal. The structure has the advantages of low insert loss, high reliability and coherence. The design method of transform and the expressions of the ridge waveguide’s impedance are presented. The result of electromagnetic simulation is also given in this paper. According to the optimum result, the test on the practical transform is carried out. The insert loss is less than 0.3 dB, and the standing wave ratio is below 1.5 dB. Better results can be obtained if the transform is optimized for specified bands. The results can satisfy the engineering requirements.