An integrated CAD/CAM system for MMIC development has been firstly realized, which consists of electron beam direct drawing, microwave circuit simulator, pattern generator and RF &DC on-wafer automatic measurement subsystems, connected through an Ethernet LAN. The system can develop not only new MMICs and their element devices, but also their accurate simulation models quickly and efficiently. Preliminary successful applications of this system have been demonstrated by DC-HFET with a 0.25 µm T-shaped gate electrode and MMIC low-noise amplifiers operating at X- and L-bands.