Droplet oscillation is an intriguing study that has inspired researches in many fields, and it becomes increasingly important on biochip applications. In this study, amplitude and phase spectrums and beat phenomenon of water droplet oscillation driven by ac electrowetting are studied using frequency scanning method. It is observed by experiment that at resonant frequencies of water droplets phase differences between the driving voltage and the droplet motion are ±90°. In addition, near resonant frequencies beat phenomenon of water droplet oscillation is also observed by experiment.