This paper presents a new waveguide-based spatial power combining amplifier at millimeter-wave frequency. In this amplifier, a waveguide-microstrip spatial power dividing/combining network is adopted. A four-way broadband low loss symmetrical power dividing/combining is achieved at millimeter-wave frequency with an easy manufacture processing and convenience of integrating solid-state high power devices. The measured output power of this amplifier is more than 43.4 dBm in 29～31 GHz, and the corresponding power combining efficiency is more than 80%. The obtained output power is more than 25 W in 30~30.6 GHz.