In this paper, we present a Ka-band two-stage power amplifier with flip-chip assembled 0.15 μm-gate pHEMTs. With characterized 0.15 μ-gate GaAs pHEMT and consideration of the Au-Sn pillar bump transition, the GaAs pHEMTs were flip-chip assembled on Al2O3 substrate where the passive components and coplanar waveguide (CPW) connection were designed and fabricated. The measured maximum S21 at 38.56 GHz is 20.7 dB with bias conditions of VD, = 3V, VG1 = -0.2V, VG2 = -0.15V, IDI = 52mA, and ID2 = 110mA. The output power at 38 GHz is larger than 15 dBm with linear gain of 15 dB. The load-pull measurement was also carried out on the amplifier to demonstrate the difference from 50Ω load measurement. The difference shows the capability of pHEMT model used at ka-band for delivering the maximum power.