SiGe Radio Frequency Integrated Circuits have been designed and fabricated for use in the X-band Si-based multi-function chip, which is known as a core-chip for a Transmit/Receive module of a phased array radar systems. In this paper, six kinds of Radio Frequency Integrated Circuits will be presented such as a power amplifier, a digital serial-to-parallel converter, a Single-Pole-Double-Throw switch, an 8 dB attenuator, a 16 dB attenuator, and a 45 degree phase shifter. The power amplifier based on a cascode configuration using SiGe HBTs shows the Radio Frequency performance of the 12 dB gain and 19 dBm P1dB. The other five chips have been designed using 0.25 um Complementary Metal-Oxide Semiconductor technology. The digital serial-to-parallel converter shows the perfect functional operation to 20 MHz clock. The Single-Pole-Double-Throw switch, the 8 dB and 16 dB attenuations, and the 45 degree phase shifter show good Radio Frequency performances. The most of chip sizes are very small to be lower than 1 mm2. These chip sets may make an attractive solution for a high power X-band multi-function chip for a T/R module of a phased array radar system.