This paper presents a compact and low-profile GaN power amplifier using interposer-based monolithic microwave integrated circuit (iMMIC) technology. The power amplifier based on iMMIC technology consists of a fully embedded discrete GaN HEMT power device in a silicon interposer with a cavity and several passive components such as resistors, inductors, capacitors, and transmission lines for matching networks using silicon integrated passive device (IPD) process. By adopting known good die and semiconductor process, this technology can improve yield and provide compact, low-profile solution compared to conventional hybrid MIC technology. Also, the standard silicon process can drive this solution to cost down compared to high cost GaN MMIC process. The size of the power amplifier using iMMIC technology is only 1.6 mm × 2.8 mm × 0.12 mm. The measured result shows that the iMMIC power amplifier has a P3dB of more than 37.7 dBm with a drain efficiency of 53% at 8 GHz.