An eight-way power amplifier at Ka-band was designed, fabricated and measured. Measured results show that the maximum combined output power is 4.2W (continuous wave) at 26.5GHz. The power combining efficiency is higher than 60% over the interested frequency range (25.1～28.4GHz) and it can reach to 72.5% at 26.4GHz. The foundational cell of this amplifier is a double antipodal flexible substrate integrated waveguide (FSIW) including symmetrical dove-tail taper transition structure. A number of dove-tail tapers of these cells are stacked together and plugged along narrow wall into a couple of normal waveguides, a broadband power divider/combiner is then simply implemented with low insertion loss. The results demonstrate that this technique can be applied in broadband power combining amplifiers at millimeter-wave frequencies.