This paper describes a newly developed prototype engine mount which is electronically controlled for variable spring and damping force. The controllability was accomplished by applying Electro-Rheological (ER) fluid whose apparent viscosity can be varied by applied electric field strength. Main attractive characteristics of ER fluid are wide variation of its apparent viscosity and quick response time, and one of the promising applications is a controllable damper. In the present paper, a prototype engine mount was constructed and its typical performance was investigated, where all the experiments were conducted under constant vibration amplitude to examine the amplitude dependence of the ER characteristics on ER devices. On the basis of the experimental results, numerical simulations of vibration control with ER engine mount and an adaptive neural-net control system were conducted. It is concluded that the stiffness as well as damping capacity of the present engine mount can be varied by controlling the applied electric field strength. Furthermore, an idealized model was proposed for the present type of engine mount and numerical simulations with the idealized model show the possibility of engine noise reduction with the present control system.