Variable displacement technology developed to improve the fuel efficiency engine system is going to be widely adopted because fossil fuels are in danger of running out. Variable displacement is the technology which makes 2 or 3 cylinders be deactivated when driving at fixed speed or on a downhill road. If the chassis vibration originated from engine oscillation is changed abruptly due to intentional misfiring of cylinders, it can give driver and passenger unpleasant feeling. So we have to make the vibration characteristics be kept unchanged during engine's operational mode change. We propose an adaptive control algorithm to generate the desired power spectrum modifying the conventional Filtered-x LMS algorithm for active engine mount control. Desired power spectrum may mean the vibration characteristics, such as the information of the frequency and magnitude, when full cylinders are used.