Abstract: In order to improve the output power of Ka band power amplifier, this paper presents a novel waveguide-based spatial power combing amplifier at millimeter-wave frequency. A magic T-power combiner and a waveguide-microstrip transfer are adopted in the waveguide spatial power dividing/combining network. And a four-way broadband low loss symmetrical power combining is achieved at millimeter-wave frequency. In the millimeter-wave power combining amplifier, the corresponding power combining efficiency is more than 82% in 25-29GHz, and more than 86% in 25-28GHz.