A high-gain amplifier module with integrated waveguide (WG) has been presented for millimeter wave applications. In order to improve the isolation between the amplification stages in the multi-stage amplifier module, an isolated WG is integrated into the module case. It is possible to effectively suppress the oscillation occurring in the high gain stage. Microstrip line (MSL)-to-WG transitions are designed and fabricated on a 5 mil thick RT5880 substrate for interconnection of the isolated WG, input/output WG and amplifier PCB in a cascaded two-stage high gain amplifier module. The transition loss of −0.44 dB is obtained at 40 GHz and return-loss (S 11) bandwidth below −10 dB is from 34.1 to 50 GHz. The fabricated high-gain amplifier module shows a high gain over 39.7 dB from 38 to 41 GHz. At 38.7 GHz, its maximum gain of 44.25 dB is achieved.