A waveguide based four way millimeter-wave power combining network is studied using HFSS in this paper. The power combining network is composed of a power divider sub-network, four metal PCB cavities and a power combiner sub-network. A compact waveguide directional coupler is designed to act as the power combiner and power divider. Parameters and shapes of the PCB cavities are carefully studied to avoid the resonant modes of the cavities, and a rhombus-like PCB cavity which has good transmission coefficient is presented.