The real-time feedback control of electron density was performed in an inductively coupled plasma (ICP). A 36 GHz heterodyne interferometer was adopted as the sensor for electron density measurement. The actuator was rf power which drove the ICP antenna. The results show that the electron density in an ICP system is a type 0 system. Therefore, a proportional-integral controller is necessary to eliminate steady-state error. A prefilter was designed to smooth the desired step change of electron density so that the variation of rf power became mild. A feedforward compensator was added to reduce the disturbance effect resulting from pressure change. The experimental results showed that the control system could quickly track the desired electron density and compensate the electron density variation resulting from pressure disturbance.